rearguard action
1. military A defensive action undertaken to protect soldiers as they retreat. (Typically carried out by the "rearguard," a unit positioned at the rear of a formation.) After the mission went sideways, the unit's heroic rearguard action saved a lot of lives.
2. An attempt to try to stop something that likely cannot be prevented. Primarily heard in UK, Australia. I know we're probably fighting a rearguard action against the board with this petition, but it's worth a try.
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