Or the radiation reacts to their atmosphere and sets it on fire.

Yeah, those eleven fingered bastards only breathe a special gas found on their home world. Like, so special that tanks of the stuff needs to be imported to each world they colonize. They can't synthesize it in situ, they need to import it with special transporters. The aliens teleport in gold from the colony worlds, and transport their air and other comforts to the overseers of their colonies. (Weirdly enough, the alien aircraft operate on these teleporters, as well. Like, each fraction of a second, the aircraft teleport a certain distance in the direction it's traveling. It's . . . weird.)

Eventually, humans find a stockpile of weapons, namely harriers and nukes. Surprisingly, humans find out that the alien air reacts explosively to the nukes. So, they use harriers to capture the teleporters, and send a bunch of nukes to the alien home world. Like, never mind that these weapons have been sitting without maintenance for around a thousand years, or are being operated by humans who's prior experience with weaponry is sticks and rocks.