Note: One of the greatest military campaigns in recent history was the Ob'enn "defeat" of the Kssthrata. The two races shared the same star system for millennia, and thanks in part to Ob'enn totalitarianism and self-indoctrination, they never could play nice. The Kssthrata would have been quite happy to be left alone to their world, but at the peak of the thousands-of-years-long conflict they had a military twice the size of that of the UNS... and it still wasn't big enough to protect them from the Ob'enn.

Finally, the Kssthrata gave up. The teraport had opened up literally millions of unexplored star systems, and as soon as a Kssthrata teraprobe returned with news of a suitable empty world, they packed everybody up and moved in.

EVERYBODY. Twenty-two billion peace-loving velociraptoroid sophonts packed up and moved from their homeworld, and managed to do so without letting the Ob'enn know they were doing it.

When the Ob'enn finally smashed Kssthrata defenses and pounded their planet's biosphere into damp gravel, there was nobody left there to care, and the Ob'enn never knew they'd been had -- at least not until they read the only-slightly-anonymized account of the largest retreat in known history in Milstrategist's Monthly.

The Kssthrata campaign against the dominant life form on their new homeworld was much less impressive, mostly since it was an oxygen-producing prokaryote.