This writeup is part of the background scenario for a lot of the stuff I've been linking from the Unfinished Stories
In the 1800's (I think) there was a nationalist movement in Italy called Italia Irridenta or 'Italy Unredeemed'. They wanted to Italy to reclaim some lands in the northeast of the peninsula. Well, this is sort of the same idea-- after Earth falls to an alien invasion, a movement is formed that calls itself Terra Irridenta. With the help of brilliant tacticians and reverse-engineering of captured alien technology, they grow from a disorganized but dedicated paramilitary into an army that just might have a chance at overthrowing the aliens.
TI are like a pan-human PLO or IRA. They're fanatical pro-human, anti-alien terrorists who will stop at nothing to 'Take Back the Earth' or at the very least make the occupation of Earth as costly as possible.
They're no comic book heroes at all, though. They have the same things wrong with them that violent paramilitary groups have today-- corruption, witch-hunts, internal fighting, forcible drafting of civilians, and siezure of civilian property 'for the greater good'. Furthermore, just like the IRA and the PLO, TI sometimes find themselves being the closest thing people have to a government. These soldiers who have been trained to fight aliens now must soften their tactics and face the more complex task of being cops, judges, and statesmen.
Obviously, the attitude of most humans toward TI is overall positive, but there are places where TI is hated. Several rival anti-alien militias have been organized, and some humans may have even entered into alliances with the aliens, thus getting labeled by TI as 'species traitors'.