I guess that would depend on the technological advances they would have made in that span (this is given that Kainan's people are interested in planets with Humans on them already). Even 1300 years ago Kainan's people demonstrate technology vastly superior to ours today.
Depending on if they were merely interested in taking over or wiping us out would make a difference. Planetary bombardment from space and then gunships that track down every last person... Some might survive but I don't think there'd be a chance of driving them off.
If they only wanted to occupy Earth, and take over and subjugate everyone then I don't think you'd see much difference than what happened in any of Earth's own imperialism attempts.
With current technology in the film, and would try to invade us then I think we would have given more resistance than what the moorween did, since they are animals, and we have the ability to make weapons to defend ourself, with tatcits and stuff. If I think if they bombard us, then surely they must have ground fources to engage us in a guerilla warfare. But we do have air defence misille or space satellites that could stop them a little