The wording of the question dictated my answer: "Become friends with them".
The reality is that an extraterrestrial army that visited Earth would be unbeatable. It would be akin to Paleolithic hunters fighting with spears against modern stealth bombers. I'm reminded of the classic Star Trek: Next Generation episode, where Data finally convinces a planet to stop trying to resist an alien empire by pointing out that they will never even see the faces of their killers.
It's not that there would be no point to use thermonuclear warheads, attempt infantry attacks, or attack with missiles and artillery. It's that negotiations to attempt to make an alliance or pursue peace would be an infinitely better strategy. If selling out would cost people slavery or the deaths of nations, I would insist that we fight and refuse to give in. But I would push for negotiation and diplomacy as much as possible, both because I don't believe in killing sentient beings of any kind and because it'd be the best strategy.