We have supersonic aircraft that can fly over drop tactical nukes and then be miles away before they hit the ground. We have nukes, Army, Navy, Air Force, SWAT, Delta Force, hell, we have bombs so accurate we could drop them through the PH UCKERS windows, that's if, they even have any (probably not).
It is in no way an accurate measurement to judge how much resistance humanity could put up against the Combine. For those of you who really should be intelligent enough to realise, it is a plot element purely used for convenience by the writers. Okay, I am surprised that you g33ks need this pointing out but from what I have seen of the oversized space slugs ('Combine') there is no way they could have defeated the military might of earth in 7 hours.