The Army was sent in but didn't make much in the way of difference (which isn't saying much for the Marvel Movie Universe US armed forces given a guy with a bow and a woman with a pair of autloaders managed to hold their own against the invaders) as mentioned in one of the newsclips.
My impression is that the National Guard units that showed up were trying to hold a perimeter and (especially
) evacuate civilians to safer places underground. They weren't swarming the portal from all sides to clear the streets of aliens, but I can think of two reasons for that.
One, there may not actually have been that many soldiers- say, a National Guard regiment
showed up, to hold a perimeter of several blocks and take care of tens of thousands of civilians.
Two, there may have been a mixup in the high command. Maybe some high-ranking Army general got orders that the Shadowy Council was trying to nuke Manhattan and stopped any more reinforcements, preventing a counterattack. Maybe the colonel or whatever on the spot decided (see 1) that evacuating civilians while the crazy flashy people fought the invaders made more sense than trying to use foot infantry with rifles to clear out an army of flying chariots and giant hover-monsters
Remember, one of the beings seen fighting the aliens is the Incredible Hulk. The army has experience with the Hulk, and most of it involves getting their butts kicked. They may be hoping that either the aliens kill the Hulk or vice versa, and they move in to finish off the survivor.
...their cream of the crop (i.e. SHIELD) thinking aircraft cannon fire could do any more than piss off The Other Guy and the pilot doing the pissing off...getting way to close after having been explicitly told not to get too close.
Uh, I think you may be wearing that cowl a little too tight.
When I saw that scene, it was obvious to me what the F-35 pilot was trying to do. He was trying
to piss off Hulk with the machine gun, so Hulk would leap out into midair to attack the plane. Then the pilot bails out and leaves Hulk grabbed onto a crashing airplane. Hulk can't fly, so that's Sir Isaac Newton's call to take over.
Sure, Hulk'll probably survive falling 20-30 thousand feet, but at least he's not the Helicarrier's problem anymore.
So... incompetence? Or just the act of a man with balls of granite? You be the judge.