In the ME2 DLC Arrival, the Reapers are revealed to be entering the galaxy near a specific Relay so that they can use that relay to get onto the network and start attacking. After we blow it up, they are then stuck at the edge of the Galaxy, proceeding much more slowly.

She says that she and her people were investigating rumors of Reaper artifacts on the fringes of the system. Kenson claims they found proof that the Reapers will arrive in the system and when they get here, they’ll use the mass relay to travel throughout the galaxy. Her team calls it the Alpha Relay and from there the Reapers can invade anywhere in the galaxy. Source

Shepard then deduces that, given the story, Kenson chose to destroy it. She confirms that. It would stop the Reapers' invasion as it would take months or even years at standard FTL speeds to reach another relay

However, at that point, there is a massive Reaper fleet, in the galaxy. How is it possible that no one knows this? Additionally, how could we lose contact with the majority of the Solar system without anyone getting any signals off?

"We just lost contact with Luna base"

Wouldn't the lunar base have some sort of long range tracking, and tell Earth command?

To summarize, since the official canon status at the end of ME2 is that the Reapers are in the galaxy and the Alliance knows about it, how could they possibly be attacked and not know about it, with all communication just stopping totally?

  • I don't think they were in the galaxy at the end of ME2. They probably built a relay in dark space- hence how they could get to the arrival relay and the Citadel. Once those two were destroyed, they'd have to hoof it. Jun 13, 2016 at 15:02
  • The reason for blowing up the Batarian system in the Arrival DLC was because they had to hoof it into the Galaxy with slower regular FTL, and that system was the location of the Mass Relay that was closest to the edge of the galaxy, so the first one they could reach. Blowing it up bought the galaxy "several months" while the Reapers then had to FTL to the next closest Relay.
    – Kelderic
    Jun 13, 2016 at 15:04
  • @AndyM- FTP? That's a novel way to travel... :) Jun 13, 2016 at 15:06
  • I've editing the question to add source quotation.
    – Kelderic
    Jun 13, 2016 at 15:11
  • To correct a big error I see in multiple answers here: I just replayed Arrival, and you 100% use the relay for a jump, just before it is destroyed. The Normandy does not need to standard FTL back to another relay.
    – Glori
    Nov 18, 2021 at 3:19

4 Answers 4


Your question is partly based on a false premise; the Alliance knew a lot more than you're giving them credit for. In fact, they were quite clear that:

  • There was a big, threatening something out in the galaxy, heading towards them; Hackett mentions this in the opening cinematic:

    Hackett: We just lost contact with two of our deep space outposts. There's something massive on long-range scanners.

    Anderson: Is this what Shepard warned us about?

    Hackett: I'd stake my life on it.

    Anderson: How long do we have?

    Hackett: Not long. I've sent word. The fleets are mobilizing.

    And Anderson tells you the same in the first in-engine cutscene:

    Shepard: What's going on? Why's everyone in such a hurry?

    Anderson: Admiral Hackett's mobilizing the fleets. I'm guessing word's made it to Alliance Command... something big's headed our way.

    Shepard: The Reapers?

    Anderson: We don't know. Not for certain.

    And the Defense Committee says it again later in that scene:

    Alliance Council: Whole colonies have gone dark. We've lost contact with everything beyond the Sol Relay.

    And it's implied by the Alliance Third Fleet war asset description:

    When the Reapers came for [Arcturus] station, Singh had already pulled his command ship, the SSV Logan, back to an ideal firing position for its mass accelerator cannons.

  • This threat was coming from batarian space, which is stated explicitly in the Fall of Earth codex entry (emphasis mine):

    The Reapers took Earth in a matter of hours. The Alliance knew the first wave would arrive from batarian space, but they were unprepared for the speed and scale of the attack.

Your follow-up question, how they could be taken by surprise to the extent they were, is much more interesting, and is obliquely answered by the Fall of Earth codex entry I quoted above: the speed and scale of the attack were totally unprecedented. I'm going to dig deeper into both of those, but there's a third contributing factor I want to spend some time on:

They didn't have a lot of intel

Although Shepard (repeatedly) points out that the "something big" is pretty obviously Reapers, Anderson says that they're not certain. There's a good follow-up question in why they weren't certain, which is something not explicitly mentioned1 but for which we can make some educated guesses:

  • Intelligence from batarian space is scarce. The The Fall of Khar'shan codex entry reveals that what information did get out was vague, and quickly shut down:

    Moments after the information minister took to the extranet and announced that unknown ships were destroying all space traffic near Khar'shan, the defense minister declared there was no reason to panic. The planet's comm buoys were destroyed next, creating an ominous silence that has persisted ever since.

    And an Alliance News Network article, posted on the BioWare blog a few days before Mass Effect 3 was released, indicates that some information came out of the batarian refugees we later see on the Citadel, but it was inconsistent and evidently hard to turn into actionable intelligence:

    According to the batarians, they are neither invaders nor defectors, but refugees. They claim a hostile species has attacked the Hegemony's fleet, bombarding Khar'shan and other batarian planets.

    With the Harsa comm buoy system crippled, communication is sporadic at best between far-flung batarians and their government on Khar'shan. Some refugees claim the attackers were Council. Others blame the geth or even the rachni.

    This amount of information is enough for the Alliance to figure that something is going on, but isn't nearly enough to tell them that "something" is the Reapers. And, of course, the state of diplomatic relations between the Alliance and the Batarian Hegemony makes it impossible for the Alliance to get first-hand intelligence from that region.

  • The FTL comm system is hilariously fragile. This got a brief mention in the last bullet, but deserves to be emphasized: the FTL communications system relies on strategically-placed comm buoys, which are basically miniature mass relays, to transmit over long distances; without them, you're still limited by the speed of light. This is all described in the Communications codex entry from the first game, which also notes that comm buoys are typically disrupted during wartime, for obvious reasons:

    Comm buoys are maintained in patterns built outward from each mass relay. The buoys are little more than a cluster of primitive, miniature mass relays. Each individual buoy is connected to a partner on another buoy in the network, forming a corridor of low-mass space.


    With this system, the only delay is the light lag between the source or destination and the closest buoy. So long as all parties remain within half a light-second (150,000 km) of buoys, seamless real time communications are possible.


    During wartime, comm buoy networks are the first target of an attack. Once the network is severed, it can take anywhere from weeks to years to get a message out of a contested system.

    We know that the Reapers cut certain regions off from the comm buoy network during their invasion (Khar'shan, for example), so it's logical to assume that they'd do something similar in other systems on their way to Earth. The only ways to circumvent this would be to:

    • Get a signal out before the Reapers could destroy the buoys
    • Send a physical courier ship, which runs into the immediate problem of that ship being easy picking for the Reapers, to say nothing of trying to outrun them (I'll cover Reaper speed shortly)
    • Use a QEC, which are still rare by the time of the attack on Earth; in the whole series we only see a handful of people using one
  • Sensors have technological limitations that prevent more useful intel. This substantially explains why nobody (even the batarians) knew the Reapers were in the galaxy immediately following the events of Arrival, when they were almost certainly making all speed towards the Vular system: sensors in Mass Effect don't work the way they do in, say, Star Wars; there are quasi-realistic limitations on what they're able to sense. These limitations are discussed in the Starships: Sensors codex entry from the very first game (emphasis mine):

    "Light lag" prevents sensing in real time at great distances. A ship firing its thrusters at the Charon Relay can be easily detected from Earth, 5.75 light-hours (six billion kilometers) away, but Earth will only see the event five hours and 45 minutes after it occurs. Due to the light-speed limit, defenders can't see enemies coming until they have already arrived.


    [Long-range] sensors include visual, thermographic, and radio detectors that watch and listen for objects in space. A powered ship emits a great deal of energy; the heat of the life support systems; the radiation given off by power plants and electrical equipment; the exhaust of the thrusters. Starships stand out plainly against the near-absolute zero background of space. [Long-range] sensors can be used during FTL travel, but incoming data is significantly distorted by the effect of the mass effect envelope and Doppler shift.

    [Short-range] sensors are radars and high resolution ladars (LAser Detection And Ranging) that emit a "ping" of energy and "listen" for return signals. Ladars have a narrower field of view than radar, but ladar resolution allows images of detected objects to be assembled. [Short-range] sensors are useless when a ship is moving at FTL speeds.

    Basically the only thing long-range sensors even could tell the Alliance is that there are a lot of energy signatures heading towards them, and all kinds of sensors are stymied by ships travelling at FTL speeds, as the Reapers would have been for most of the trip from Khar'shan to Earth.

The Alliance underestimated the speed of the attack

The most obvious problem is the plain, raw speed of the Reapers. We don't really know exactly how fast they are, but the Reaper Capabilities codex entry from the third game gives an estimated speed of 1.25 lightyears per hour:

The Reapers' thrusters and FTL drives appear to propel them at more than twice the speed of Citadel ships. Estimates of their location in dark space suggest they can travel nearly 30 light-years in a 24-hour period.

At that speed they could travel from Pluto to Earth in about two seconds2, and cross the length of the Asgard3 system in less than one.

Of course, we don't know that the Reapers are moving this fast during their invasion, but they're obviously going pretty dang fast. In the opening cutscene, there's about nine seconds between hearing that the Alliance has lost contact with the moon and receiving the video from London; in those nine seconds, the Reapers have traveled at most 400,000 kilometers and done severe damage to London (we can already see plenty of destruction in the background of that brief video). It's reasonable to assume that they'd have been moving even faster when travelling between systems, since those nine seconds also account for some time travelling at sublight speeds (to make planetfall on Earth).

This speed, ultimately, is a large part of how the Reapers were able to get to the moon so much quicker than Anderson had expected: they have more raw thruster power than anything the Alliance has ever encountered before (and recall that Sovereign was clearly not demonstrating his full capabilities in the first game; in the Battle of the Citadel cutscene, he's moving as quickly as, if not slower than, his geth armada). The other reason is the question of scale, which I'll come back to.

What's more, the Reapers made better time than the Alliance was expecting; the Fall of Earth codex entry reveals that the Reapers skipped straight past the intermediate systems on their way to Earth, entirely bypassing the Alliance's first two lines of defense:

The Reapers bypassed the Sixth and Seventh Fleets at Terra Nova and Eden Prime, flying straight from relay to relay where they could neither be tracked nor intercepted. The tactic was unexpected, since the navies of organic species would never risk coming out of FTL within combat range or leaving enemies at their backs to threaten supply lines.

This also illustrates that the Alliance failed to understand their enemy, which left them more vulnerable than they otherwise might have been; it's not hard to imagine how the Reapers blew past the Alliance's defences, when large portions of the defence was focused on defending against the wrong kind of attack.

The Alliance underestimated the scale of the attack

Radhil has already admirably covered blitz tactics, but I want to mention some specific factors from the series. First, as the Fall of Earth codex entry mentions, the Reapers sent a lot of ships to Earth (emphasis mine):

At Arcturus Station, more than a dozen Reaper capital ships engaged the Alliance's Second, Third, and Fifth Fleets. This was mere screening for the main force. Dozens more capital ships continued through the Charon Relay, where the First Fleet had been lying in wait but was soon destroyed. The Fourth Fleet, near Earth, had a few minutes of advance warning. It stood no better chance.

We have ample evidence that the Reapers completely outclass any organic race, and can pretty trivially tear through anything short of supremely overwhelming force; if caught off-guard, as the Alliance was, it's not hard to imagine several dozen Reaper capital ships absolutely shredding a fleet.

What's more, as that codex entry points out, the Reapers were perfectly willing to divide their forces to get at Earth quicker; if they did so at Arcturus, it seems reasonable to assume that they'd have done it again after getting through the Charon relay, and possibly again at the moon. It's worth noting that they pulled a similar strategy against the turians, who were admiteedly much better-prepared for the attack4, as mentioned in the Battle of Palaven codex entry:

The Reapers countered instantly. Their destroyers performed a jump of their own to the skies above Palaven, beginning orbital strikes on turian cities. The turians, forced to defend the planet, found themselves in a pitched battle far from the relay, from which emerged a seemingly endless line of Reaper ships.

If that's what they did, a force of Reapers could easily have been most of the way to Earth before anyone in the Fourth Fleet could even manage to get a signal off.

All of this being the case, I'm actually more surprised the Alliance had any warning, though (again) we know that they did. We might imagine that various ships managed to get some signals off before being outrun or destroyed by the Reapers, but the information in the game isn't quite detailed enough for us to confirm that.

1 It's even more baffling if you didn't do the Bahak mission. In that version of the story5, Hackett sends some marines to investigate Kenson's disappearance, and they decide to blow up the relay to stall the Reapers. There's an update to the 103rd Marine Division war asset description in this case, which reveals that the Alliance was able to recover some evidence of this mission:

Admiral Hackett dispatched marines to the planet Aratoht to rescue a deep cover agent, Dr. Amanda Kenson. The teams were killed in an explosion that wiped out both the colony and the system's relay. The Alliance spent weeks piecing together scattered radio transmissions, learning that the marines felt they had no choice but to send an asteroid into the relay to prevent invasion by the Reapers. While it bought the Alliance some time, the men and women lost on the mission were a severe blow to the 103rd Marine Corps.

How the Alliance can still doubt the presence of Reapers with this information strains credulity; there are a few mental contortions we can do to make this more sensible, but there's no additional justification given in the game

2 Unfortunately, Wolfram Alpha won't let me account for the maximum distance between the Earth and Pluto, which Space.com gives as about 7.5 billion kilometers; at that distance, it would take about a half-second longer

3 Using the orbital distance of Loki (given in the planet's description in the third game), the outer-most planet of the system, as a boundary; Loki's orbital distance is only 6.8 AU, which makes this a pretty small system. The Utopia system is much larger, and (using Xanadu's orbit as the boundary) would take about 9 seconds to cross.

Using planetary orbits as a boundary isn't consistent with either the game (which shows a farther boundary for each system, often some distance from the outermost planet) or real life (where the boundary is vaguely defined but impossible to calculate in-game); the actual numbers are invariably going to be larger than what I've calculated, but to a first approximation it's as good as we're going to get

4 The reason for the discrepancy is, in part, because the Reapers attacked Palaven somewhat later than they did Earth, and partly because they first assaulted the turian colony world of Taetrus and then launched a propaganda campaign aimed at demoralizing them; this is discussed in the Fall of Taetrus codex entry:

By the time Taetrus went dark, the turians had already learned that the batarians and humans were under attack. [...] The Reapers emerged victorious from the relay and began broadcasting a signal to turian comm buoys--images of Vallum, Taetrus's capital, once again a smoking wreck.

5 Since a lot of the comments on this question talk about what happened "canonically", I think this point is worth emphasizing: very few of the events in the Mass Effect games can be considered objectively "canon"; this was actually a stated goal of series director Casey Hudson, as he said in a 2012 interview:

Hudson: We have a rule in our franchise that there is no canon. You as a player decide what your story is.

BioWare took great pains to ensure that the game still made sense regardless of what choices you make as a player; so if you don't do a particular mission, there will either be a "default" choice (for example, not doing the Bring Down the Sky DLC defaults to an asteroid crashing into Terra Nova) or a sensible alternative (as with Arrival, mentioned in a previous footnote, or with Udina becoming Councillor in the third game).

Basically, if you can make a choice (or if your choice affects an NPC's choice), there is no such thing as an objectively canon choice; there's only the choice that's canon in that particular playthrough


The purpose of a blitz attack is to overwhelm your opponent before they can respond.

They did enter the galaxy around Baatarian space, as the Arrival story suggested they would. They spent a brief amount of time building up forces at the expense of the locals. Then when they moved, they didn't go for the closest system. They didn't establish a boundary around their space and expand outward. They didn't behave in anything resembling an established military doctrine. They just took most of their force, skipped past every other system, and they hit Earth.

They hit Earth hard.

The Reapers had enough information to know Earth (or more specificially, Shephard) would be the strongest threat against them. They had every bit of information that Harbinger had found over the time the Collectors were preying on human outposts. They had everything they needed to just blitz. Earth was on alert, but that's not the same as knowing when the hit was coming.

Getting advance warning back to Earth would require someone to know the attack was coming, and keep an open channel up long enough to get that warning through. Given that a) the Reapers have plenty of ubertech to jam standard comm channels with, b) they knew exactly what to hit and where to take out command and communication centers, and c) they struck with overwhelming force specifically so that Earth would not have time to prepare, it's not really any surprise the first warning they got was the orbital bombardment.

  • That makes sense partially, however, the next closest Relays to the destroyed one would be known, and wouldn't they be garrisoned? In the opening dialog of ME3, and in the post mission Arrival dialog, it's known to Alliance authorities what happened and therefore that the Reapers are in the Galaxy in that area, so logically they'd move to the next closest Relay to get onto the system.
    – Kelderic
    Jun 13, 2016 at 15:57
  • I guess thinking logically, if regular FTP is slower than Relays, it would then mean that that portion of the Galaxy is now harder to get into. It would takes months of FTL travel to get from the closest operable Relay in to check the area. So if the council started investigating, it would take months. Theoretically, if ships could pass each other in FTL without realizing, the Reapers could get out and to the next Relays before anyone was aware.
    – Kelderic
    Jun 13, 2016 at 16:02
  • @AndyM - It was Bataarian space. They rejected the Council before the first game, spent most of the games being terrorist mooks / pirates, and would've considered any of the Council-aligned races setting up a defensive post against "mythical Reavers" as an invasion, so any human/Council forces would have to fight... just to get to fight more. The destruction of the relay in their space was considered a war crime. Well, up until the myth looked out of the void and said "Boo",
    – Radhil
    Jun 13, 2016 at 16:03
  • All of this breaks entirely though, because the Normandy had to use regular FTL to get out, which means that the Normandy should have gotten back to the Galaxy only hours ahead of the Reaper fleet (assuming their regular FTL speeds aren't any greater than the Normandy's, which I find hard to believe). That means that the only way the plot makes sense is if Arrival happens after the Collector base. The Normandy goes to investigate, is out of contact for months, arrives and goes straight to Earth, with Reaper fleet hours behind.
    – Kelderic
    Jun 13, 2016 at 16:05
  • 1
    @AndyM - if it helps, I believe canonically Arrival does happen after the Collector suicide mission. Shepherd turns himself and Normandy in to Earth immediately after, although I don't know how long he was imprisoned for before the Reaper blitz.
    – Radhil
    Jun 13, 2016 at 16:15

So based on comments and on Radhil's answer, I've come up with a plot series which I can't poke any holes in:


First thing we need to understand is the nature of travel in the ME universe. Ships have FTL drives, meaning they can move around the Galaxy. However, it's slow by itself. The Relays which are scattered around allow for very very fast travel. Regular FTL travel between two Relays would take several months, while a Relay assisted jump might take hours.

Reaper Plan

So the Reapers need the Relay grid to quickly move around. They were anticipating coming into the Galaxy using the Citadel Relay, which is larger than most of the network, the only one powerful enough to talk to outside of the Galaxy. When that was stopped at the end of ME1, they instead had to use slower regular FTL drive to get into the Galaxy and onto the Relay network.

They closest Relay to the edge of the galaxy happened to be in Baatarian space. This is the Relay that Shepard destroys in Arrival. The plan is to get to that Relay and then from there they are in the network. However, Shepard destroys it, which means that they have to use regular slower FTL to get to the next Relay in the network.


Here is where things get tricky, and Bioware does a bad job explaining. Normandy would be stuck using slower FTL to get back to the network, which is ignored. It's possible to complete Arrival before the Collector Base mission, and from everything I can think of, this totally breaks the plot. So for the cannon plot, we assume that Arrival happens after Collector Base, and after everything else in ME2. Cannon plot assumes Arrival is the very last thing before plot events of ME3.

So a logical plot then follows:

Plot That Makes Sense

Shepard destroys the Collector Base and proceeds to finish up small things (side quests in ME2), until he gets a message from Admiral Hacket about a potential arrival of the Reapers in Baatarian space. He uses the Relay system to go there, and ends up destroying the Relay. (Arrival plot). Normandy then jumps via regular FTL towards the next nearest Relay, with the Reaper fleet only hours behind. Luckily for us, the Reapers have never really pushed research into basic FTL speed improvement because of their reliance on the Relay network, so they aren't faster than the Normandy.

The Normandy (and Reaper fleet) are in FTL travel for several months. During that time, the Alliance, being unable to make Relay to Relay jumps to the destroyed Relay, sends a large fleet via FTL to investigate. Given the distances of space, it passed the Normandy and Reapers midway and arrives out in the area of the destroyed Relay far too late, and is unfortunately not available to help in the coming invasion of Earth. However, it is the core of the remaining human fleet later in the game. (This explains how we have a fleet at all later).

The Normandy comes out of FTL at the nearest Relay, and jumps back to Earth, and then sounds the alarm. However, only a few hours after Shepard gets to Earth, the Reapers jump into system. With a large portion of Earth's fleet gone, they easily blow through most of what remains, heading straight in system to Earth directly.

They have extremely good jamming capabilities, and communications start dying immediately. See Radhil's answer for more details on the Sol system invasion.


This chain of events makes sense, and only varies slightly from events in that it would assume that all crew members are still on Normandy at the beginning of ME3, which they aren't. Possibly, they are dropped off before Arrival begins, when Shepard returned to the Alliance unofficially after the Collector Base.

Bioware should have placed a requirement in the Arrival DLC that the mission opens up only after the Collector Base, with a note at the beginning that even though you can still fly around after this mission, that it is canonically the last mission before ME3.

  • 2
    Why does Arrival have to happen after the Collector Base, and why does Normandy need to use FTL to get back to the Relay network? The Normandy pretty clearly uses the Bahak relay to get out of the system before it's destroyed Jun 20, 2017 at 18:04
  • That's a good point. I don't know, it's been a year since I wrote this, and at the time I guess I'd forgotten about that. Will have to rethink this then.
    – Kelderic
    Jun 20, 2017 at 20:17
  • 1
    "Normandy then jumps via regular FTL towards the next nearest Relay" WTF . No , it jumps via the relay before asteroid hits it. Dude did you really play the dlc?
    – Allahjane
    Nov 12, 2017 at 21:42
  • 1
    This is impossible, for multiple reasons: 1. The cutscene at the end of Arrival clearly shows the Normandy leaving the system via the relay. 2. The codex explicitly states that Reaper FTL is twice as fast as non-Reaper FTL. 3. The opening scenes of ME3 clearly show Shepard under house arrest, and Shepard's incarceration is mentioned repeatedly by multiple characters. 4. The Normandy is not capable of surviving the destruction of a relay; it destroyed the entire system. This answer is completely wrong and I downvoted it accordingly.
    – Kevin
    Sep 21, 2018 at 22:51
  • Yeah there are definitely problems with this. I don't really remember the details of Arrival that well, since it's been so long. At the time I accepted the answer as correct, it was the only one. The other answers have been added over the past few years. When I get to a comment uter tomorrow, I may change the selected answer to one of the others.
    – Kelderic
    Sep 21, 2018 at 23:06

At the end of Mass Effect 2, the Reapers start their advance towards the galaxy.

Nobody in the galaxy knows of it.

The Alliance impounds the Normandy and sends Shepard into house arrest to keep him safe from Batarians seeking revenge on him for blowing up the relay.

Everyone on his team is sent to their homes.

But according to Arrival the relay was blown up when the Reapers were only minutes away. Which means by the time Arrival happened the Reapers had already reached the galaxy but after that they had to use FTL to get to nearest relay.

By the time Mass Effect 3 starts it's suggested that Shepard has been under house arrest for 2 full years.

Now that 2 years is likely sufficient for Reapers to reach Earth with their FTL, especially since the solar system is near the edge of the Milky Way.

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