Aliens with technology vastly superior to our own attempt to invade Earth. We put up military resistance, and, surprisingly, they retreat, leaving Earth alone. Resistance wasn't futile.
The reason for their retreat is that, with their technology, it's just as easy for them to colonize any other planet in our solar system, and those other planets don't put up resistance, so they go the lazy route.
"Colonize" might not be the best word: what they actually do to a planet is disassemble it (mine it) completely and use the entire planet's matter as raw material to build Dyson swarms. This is why (say) Jupiter isn't any more inconvenient as a colonization site than Earth.
Or perhaps they also destroy the Earth, but we mostly don't care because they do it slowly.
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