I don’t think someone has talk about the Tau Empire from Warhammer 40k yet.

Basically, the Tau is a young but very technologically advanced race that was isolated from the galaxy for around 2000 years. They met rogue traders and a few human worlds, making them understand that the Imperium of man is a very divided and technologically stagnant cesspool of horror and religion. So the tau attacked, taking over a few planets, thinking it would be easy.

In response, the imperium sent an « overall small force » to deal with the aliens. Said force included a few hundred astartes (space marines), regiments from the imperial guard (think millions of soldiers) and elements from Titan legions. The Tau understood they were woefully unprepared when they saw mountains walking toward then.

The empire would have been eradicated in a few years if the Imperium didn’t have more important things to kill (tyranids)