4

This question is mainly based on this previous question.

Quoting from Admiral Ackbar from the linked question (emphasis mine):

While the planet in question may at present be deliberately underdefended, the instant we move forces out of hiding and in its direction, the First Order will realize that we know the location of the weapon. They will mobilize everything in the vicinity to protect it. Their fleet is too large for us to fight our way through.

So knowing that:

Starkiller Base just crippled the New Republic with a giant, red beam of destruction that could not have failed to give away the position and the destructive power of their superweapon. Furthermore, the First Order must have known that before they fired the super weapon.

Why didn’t the First Order mobilize their fleet to protect the base right as they destroyed the Hosnian system?

  1. It seemed like their plan all along was to defend Starkiller Base with a large fleet.
  2. It makes complete tactical sense to defend the base so that it isn't immediately destroyed after you use it.
    • More so when you consider the first Death Star, which was destroyed by a bunch of rebel starfighters because you only defended your superweapon with a bunch of TIE Fighters.
  3. If they mobilized the fleet beforehand, they might have tipped off the New Republic. But they could have called in reinforcements the moment they fired the weapon at which point it would have been too late for the New Republic to evacuate. Maybe this is a reason?
4

Your point #3 is spot on. They didn't want to tip off the New Republic.

As to why they didn't call up the fleet right after destroying Hosnian system, theres's no direct canon proof of what they were thinking, BUT:

  • The only major military force that they would have needed to protect from was already destroyed

    Novelization explicitly mentions that New Republic fleet was destroyed there.

    Expanding outward from the explosion, a tremendous burst of heat tore through the Hosnian system’s other worlds, searing their surfaces clean of life and incidentally obliterating all settlements, installations, and outposts, as well as the hundreds of ships belonging to the Republic fleet. In its wake, the detonation left behind a blazing, spherical mass. The home of the Republic had become a new binary system: one utterly devoid of life.

    and

    “General, the Republic command — the entire Hosnian system — it’s all — gone.” He stared incredulously at his readouts.

  • The only OTHER military force that existed (the Resistance) was:

    • extremely small, not really worth worrying about

    • AND about to be destroyed as well. Snoke planned to smoke the the Resistance right after the Republic:

      Hux was clearly gratified to be the bearer of good news. “We were able to track their reconnaissance ship back to the Ileenium system. We are coordinating with our own reconnaissance craft in the area in order to lock down the specific location of their base.”
      Snoke replied with cold satisfaction. “We do not need it. Prepare the weapon. Destroy their system.”
      Collected and composed as he was, Hux was not immune to surprise. “The system? Supreme Leader, according to the most recent galographics, at least two and possibly three habitable worlds circle Ileenium. Following the destruction of the Hosnian worlds, would it not be worthwhile simply to destroy their base and claim the remainder for the Order? We will have the location of the base within a matter of hours and—”
      Snoke cut him off. “We cannot wait. Not even for hours. ...

  • Moreover, if not for a freaky sequence of events, First Order was somewhat right - the Resistance didn't have a chance to attack the planet.

    • They had a planetary shield that no ship could penetrate (unless they were insane Han Solo flying through in at lightspeed).

      A planetary shield means, any attacking ships have to bombard it for a while to breach it - which lets First Order bring in reinforcements if needed.

      Additionally, despite what Poe theorizes, I would wager they must already have at least a minimal planetary shield in place. Plainly, they can access the energy to support such a defense.” He looked at Finn, whose reply was not encouraging.
      “Yes, such a shield does exist.

    • They had strong planetary defense system including seeker missiles.

      “And engage seekers.”
      The officer hesitated. “In an atmospheric skirmish, sir, seekers will have a hard time distinguishing between our fighters and those of the enemy.”
      Hux didn’t bat an eye. “This is no time to worry about collateral damage.” His voice was steely. “Give the order.”

      and

      A telltale on his console began demanding attention. Flicking his attention to the attendant monitor, his eyes widened.
      Seekers. Hundreds of seekers, rising from launch batteries concealed beneath the soil and snow. Rising toward him and his fellow pilots, giving them little room to maneuver—or escape.

    • the Resistance military leaders fully acknowledged that their attack had no chance at all

      We weren’t prepared for anything like this,” Admiral Statura muttered. “Our pilots will be annihilated.”


SUMMARY

  • Nobody had any forces worth defending against anyway

  • If they did, and attacked, they had a planetary shield which would stop any attackers long enough for reinforcements to arrive

  • I find it difficult to believe that the entire New Republic fleet was in the Hosnian system. Were there really no other military forces in the entire galaxy? How can the entire galaxy be safeguarded/policed from 1 system? – Kevin Dec 25 '15 at 18:49
  • @Kevin - the Command definitely was. And NR wasn't exactly a military powerhouse, according to Leia's musings in novelization and Visual Dictionary – DVK-on-Ahch-To Dec 25 '15 at 18:50
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    @Kevin - better? – DVK-on-Ahch-To Dec 25 '15 at 19:08
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    @JakeGould - because it was, as the answer says, The only major military force worth protecting against. – DVK-on-Ahch-To Dec 25 '15 at 19:09
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    Ahh, okay. Read that wrong. That known you would think one measly shot—in the montage of planet-side people watching their own destruction—showing a familiar array of Rebel/Resistance cruisers being wiped out would clear that whole mess up? Nope. Boneheaded story-telling in the film. – JakeGould Dec 25 '15 at 19:22

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