I'm hoping that that gets explained in the (if they have them (and please let them have them (because I can't imagine this series gaining any headway if all they do is talk about what happened, in expository format))) pre-war(?) flashbacks.
Of course, they mention that one of their groups essentially got themselves nuked for using an AT4, of all things, and I would imagine that any surviving personnel would have attempted to grab gear like that, thus making themselves ripe targets.
As for the NORAD thing: There's no telling how advanced the aliens' weaponry is (though they seem to rely on kinetic force, for their ground troops), and the previously mentioned 'nuke' was declared in-show as 'not being their biggest'. I would imagine that they have some sort of ground penetrating bomb, similar to the Grand Slam.
Speaking about their capabilities, I can't help but feel a little underwhelmed at the lack of power their mechs display. :/