Techincally all the post-Avengers movie suffer this because there solo stories
Well, Thor is stuck restoring order until the end of Dark World. And even with him on Earth following that, he likely was spending all his focus on finding the staff given to Loki (and being in Europe in general given that's where all his friends are staying), making him unlikely to just drop by in time for an Amwrican crisis.
Tony was (at least in theory) out of the super hero game after blowing up all his suits at the end of IM3 (but that was apparently not even a speed bump to him going back on his word and building a small armada of Iron Man drones and suits prior to Age of Ultron).
Hulk is... well, he's a private guy who doesn't show up without an expressed invite.
To me, War Machine was the only character who should have, without a doubt, been on the scene for a huge military operation in the capital of the military he serves in.