Our only option would be become friends with them. Assuming they made it to earth, that would mean technologically they would be far more advanced than we are, so in a war of any type the odds are humans would lose.
Think of it this way, we go back in time with our technology, to say Roman time what chance would the whole Roman army have (450,000 at peak in 211 AD) against one Division of well armed troops, with helicopters, jets, tanks, bombs, rockets, etc. Odds are those 450,000 Romans would have a hard time against one Brigade, even without the helicopter, jets, etc. That's the position we'd be in, we'd be the Romans, about all we could do is say "can I buy you a beer, buddy?"