they don't need to invade. they can bomb us with asteroids on the order of tens of km across. we're talking outer space here. if aliens could come here, they'll probably have a supercomputer. with a supercomputer, they can calculate trajectories for an asteroid 100 km across with a rocket strapped to it and the earth, and we can neither move the earth how we want it to, nor can we move an asteroid 100 km across.
they get 20 asteroids 100 km across and toss them at earth with even low tech rockets. they fill the rockets with refined hydrocarbons off Titan. game over. or they can refine asteroids into a block of metal and launch it at us with a railgun (if they could come here, they can make a railgun). there is absolutely nothing we can do about it if aliens come.
The best way of dealing with an alien invasion is to bow down to our new overlords and hope that they don't want Earth to live on. If they want us to send tributes to participate in an annual fight to the death or to hunt us for fun, we can negotiate something.