A ditch or fortified position dug for defense, or the state of being deeply established and hard to change.
From entrench + -ment suffix creating a noun; combines military and figurative meanings since the 1600s.
Military trenches in WWI created literal entrenchments that sometimes lasted months, but the psychological entrenchment—soldiers' belief that the war was unwinnable—lasted years and fundamentally changed how nations thought about conflict.
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