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With reference to this question the most upvoted answer says that in Avengers: Endgame the plan was to return the

Stones back to their timelines and not snap Thanos out of existence.

But how does this solve the problem when Thanos will continue to exist and still cause harm as he has access to the

Pym particles and can potentially travel back to retrieve the returned Stones. Isn't the only way to end this would be Iron Man making the sacrifice?

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Because when the Infinity Stones are returned to their time lines the alternate reality where that Thanos and his army are from will cease to exist.

The Ancient One: The Infinity Stones create what you experience as the flow of time. Remove one of the stones, and that flow splits. Now this may benefit your reality. But my new one, not so much. In this new branched reality, without our chief weapon against the forces of darkness, our world will be overrun. Millions will suffer. So, tell me, doctor, can your science prevent all that?

Banner: No. But we can erase it. Because once we're done with the stones, we can return each one into its own timeline at the moment it was taken. So, chronologically... In that reality.... it never left.

Avengers: Endgame

Remember the scene where they have this conversation there is a diagram in the air, when the Ancient One removes one of the Infinity Stones a new time line shows up on said diagram but when Banner puts it back again the time line is erased.

This Thanos, his Children and the armies are from an alternate reality where the Power and Soul Stones were taken from. So when they put the Stones back into 2014 the Thanos and his forces would have been erased.

It's also worth noting, as Nathan K says in a comment, that the plan also stops the immediate problem of Thanos being able to get his hands on the Infinity Stones and snapping. They are removing them from harm's way.

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    And before someone comes and says "that's not how it works!" That depends on who you ask, the directors would disagree but the writers would agree. However, all that matters is that in universe this is what the characters believed would happen. – TheLethalCarrot May 14 at 9:33
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    And if you prefer the directors' interpretation-- Even if those other timelines do continue to exist, taking the stones through the quantum portal does solve the immediate problem by getting them out of Thanos's reach for the moment. Maybe this would have led to Thanos pursuing them through time and eventually catching them, or maybe he would have turned his weapons on the Earth in frustration. In either case, it's one of Dr. Strange's loss scenarios. But only he knew that at the time. – Nathan K. May 14 at 13:45
  • No, it doesn't matter what the characters in-universe believe it is. They are not all knowing. The only thing that matters is how it IS in-universe. To summarize the one timeline theory: 1. War Machine removes the stone. 2. Captain America returns the stone. 3. Timeline ceases to exist (Cap somehow survives, Quill who got knocked out starts dancing again) 4. Thanos travels from non-existence into main timeline. --- There are so many flaws in this, I don't understand why anybody (especially the writers) would prefer this against the multiverse theory. – Chris May 16 at 10:51
  • @Chris Except this is how it is in universe. The characters all say this is how it works. The writers say it is how it works and they wrote the thing! You're missing something here, that timeline where the stone is removed is an alternate timeline. Quill does not start dancing again because that timeline is erased, it doesn't exist anymore. – TheLethalCarrot May 16 at 10:54
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    @Bobson There's somewhat of a mention in that Quill is searching for Gamora at the end. It might be because she disappeared or it might be because she left at the end of the battle we don't know, it's left intentionally ambiguous. – TheLethalCarrot May 22 at 12:01
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The question asks about the original plan, so...

According to the original plan, there would be one and only one timeline. In this timeline, the sequence of events would have been:

  1. Infinity stones are "borrowed" and immediately returned.
  2. Thanos gets his hands on all of the infinity stones (i.e. Infinity War).
  3. "The Snap".
  4. Thanos destroys infinity stones.
  5. Thor kills Thanos.
  6. Infinity stones arrive from the past.
  7. Undo "The Snap".
  8. Infinity stones are returned to the past.

Past-Thanos following the Avengers back to their present was not part of the plan. If that hadn't happened, then — from the perspective of "the past" from which the stones were borrowed — nothing would have changed. Thanos would have eventually gotten the stones as he did "before", "The Snap" would have happened, and it would have been undone as in Endgame, because it would have all been one timeline.

Presumably, Thanos finding future-Nebula created a second timeline. When he subsequently invaded the Endgame timeline and was dusted, he would have ceased to exist in his own timeline¹, which presumably means that timeline still has infinity stones, while the Endgame timeline does not (unless someone created a second set off-screen before returning the "past" infinity stones).

(¹ ...or if he'd won and just not returned to his own timeline. Being dusted ensured that he couldn't return.)

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  • "The question asks about the original plan, so..." well no actually it specifically asks about the plan that was made up during the final battle. – TheLethalCarrot May 16 at 10:34
  • Also the 2014 Thanos in 2023 is an alternate reality Thanos that would have been erased had Tony not snapped anyway when the Power and Soul Stones were returned to 2014 thus erasing the timeline. – TheLethalCarrot May 16 at 10:36
  • I don't think that's true; he may have left his timeline after the stones were borrowed, but the events that led to him doing so occurred before that. It seems plausible that every time an avenger "inserted" into the past, that became a new timeline... one that didn't get ruined (as would have happened if the stones weren't returned), perhaps, but still new. – Matthew May 24 at 21:53
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tl;dr - There was no way (a debatable point) to undo the events of the timeline as we saw it, but they could sneak over to other timelines and fix the majority of the effects of The Snap.

The main problem is the somewhat confusing theory of time travel that the movies used, which is based on at least A version of time travel the comics have used.

According to the explanation, when you travel in time, you do not travel into your own past, but into an alternate timeline in which your appearance took place. So taking the stones out of the timeline does not erase their existence in "our present", and does not negate Thanos being able to retrieve them.

So the recovery missions generate several new timelines where each of the stones no longer exist where they did in the main timeline.

But by returning the stones almost immediately after they were removed, the effect on said timelines is virtually nil. So for example, the Ancient One only loses the use of the Time Stone for a brief period, not long enough to fail to defend the Sanctum.

Thanos' time trip generates another timeline, one where he simply and mysteriously blinked out of existence. That timeline never saw him collect the stones, and never suffered through The Snappening. The Thanos we saw at the end of the film was not the same Thanos we saw at the beginning, but one from a divergent timeline.

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  • I have to say any time a writer uses time travel as a Deus ex machina, I lose a little respect. Maybe a lot... – JavaMikeMoore May 15 at 20:11
  • "Thanos' time trip generates another timeline" correct, "That timeline never saw him collect the stones" correct but for the wrong reasons, that timeline is erased when Steve returns the stones. You're sort of right with your explanation but for the wrong reasons if I'm reading this right. – TheLethalCarrot May 16 at 10:39

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