Generally, as long as you have at least one ship on your fleets, the fleet cannot be taken from you. There is, however, one exception: the Raider's Commandeer attack!

If you encounter a Raider at a world, and their ships on fleets outnumber your ships on fleets by more than three to one, the Raider will automatically capture all of your fleets, along with all of the ships, artifacts, and metal on them. The Raider calls this Commandeering your fleets.

This special capture happens at the end of the round, after all normal combat.

If there are multiple players with fleets at a world, the Raider must outnumber all of them by more than three-to-one, including the Raider's own allies. The Raider will not commandeer their ally's fleets, but they still count against them in the three-to-one ratio. Similarly, if any of the Raider's fleets are declared At Peace, they will not participate in the commandeer and thus will not count towards the three-to-one ratio; but the Raider does have to outnumber any other fleets at the world, even those that are At Peace.

Commandeer involves only fleets in orbit; I-Ships and P-Ships cannot be commandeered, and these ships do not count toward or against the three-to-one ratio.