Ever play Master of Orion 2?

You'd be happily expanding your interstellar empire, technological progress coming along nicely, you've opened up trading relations with the more peaceable neighbours, fought a war or two with the more warlike ones who are now paying you reparations, your fleet has recently defeated a dreadful space monster and opened up a huge rich Gaia world for settlement, everything's rosy.

Then an Antaran fleet turns up out of nowhere heading straight for your main industrial centre.

OUCH OUCH OUCH OUCH OUCH.

Mind you, if you were smart about things, you'd focus your research on infantry equipment and build a task force ahead of time with lots of assault shuttles. Capture even a single Antaran scout ship intact, get it home and reverse-engineer it, and you could have xentronium armour, the damper field and the particle beam _really_ early in the game.

I used to experiment in the late game to see how small a fleet I could send to conquer Antares. I seem to remember I once managed it with one cruiser.