Retrenchement (spelled retranchement in French) means to reduce a portion off off or to supress a portion of something.

This is in addition to the military use. However in French, a retrenchement is not only a trench - or hole in the ground - it is any obstacle behind which one takes refuge. So you can have a retrenchement made of trees, bricks, wood or sandbags and a "retranchement creusé" (the trench).

There is also the French expression "pousser quelqu'un à son dernier retranchement" or "push somebody to his last retrenchement," which means pushing somebody to the end by refuting his ultimate arguments.