Revanchism is a political doctrine that aims to aggressively reclaim lost territory. Some experts describe Russia's 2022 invasion of Ukraine as an example of revanchism.
When a country practices revanchism, it's trying to get revenge by taking back territory lost in a war or social movement. The policy almost always involves another military conflict, and it's motivated by retribution as much as political or economic aims. The French root of revanchism is revanche, or "revenge," and the word was first used politically after the Franco-German War, when French nationalists were determined to reclaim territory that had been lost to Germany.