What the heck makes you think they are interested in a ground war? If they drop a dinosoaur-killing asteroid on us, how much resistance would you expect in a year? 90% of the population would be dead. We would have crap for manufacturing abilities.
Any surviving humans would be target practice, and nothing more. Yeah, some few folks would get a shot back, but not enough to matter.
If an alien entity wants to destroy us it would actually take a relatively small amount of delta-v applied to any number of objects in the solar system if they aren't pressed for time.
Dropping an asteroid would make the planet useless for a long while. It makes little sense if you want to take over a functioning world.