The Martians' weakness is inadvertently discovered.
Martians are intelligent carbon-based life forms, similar to Humans, but instead of breathing oxygen they breathe nitrogen. They're evil, but not because they want to; it is just that they were tired of living on Mars, they felt they needed a healthier planet and the healthiest and closest one was Earth, the perfect planet for them. So they came to Earth, beginning with a peaceful approach, but when they saw a pigeon they immediately annihilated it with their ray guns and started attacking people, and the war between Mars and Earth was started. At first, the USA president thought that the reason why the Martians were attacking was because they had a cultural misunderstanding with the earthlings. But he later noticed that wasn't the reason. Just when the aliens were almost conquering the entire Earth, an old lady was listening yodeling (specifically, Slim Whitman's Indian Love Song) and noticed that the Martians couldn't stand hearing this kind of music, as it caused their heads to explode. And thanks to the music all Martians were killed and Earth was safe again. Maybe some Martians survived, just a few of them probably survived, but now they are so scared of humans that they don't dare to approach to Earth.