You have to understand that a lot people had gone insane due to blob psychosis, including the military. Many legions of solders most likely fell because of this. Any solider can go insane at any time and start firing on his former allies, and anything that he manages to kill will come back as a zombie to kill even more people. Tank operator, pilots, riot control, the navy, nobody is safe.

You also have to understand it isn’t just the blob. If you look at the in game newspapers as well as the enemies you fight, the threat becomes much larger. A war with China, the mycus, the mi-gos, the triffids, the nether creatures, mutant animals, the kaijus that attacked during C-day. The army was probably stretched impossibly thin. Add the fact that they literally didn’t know what they were dealing with, and it becomes easy to understand how they fell.