Annex means to add territory to a country or state, usually by taking it from somewhere else. When a nation annexes land, it claims that territory as its own and brings it under its government's control. This might happen through war, political agreements, or by force.

Throughout history, powerful empires and nations have annexed neighboring lands to expand their territory. For example, the United States annexed Texas in 1845 after Texas had been an independent republic. Russia annexed Crimea from Ukraine in 2014, which most other countries considered illegal. Ancient Rome constantly annexed new territories as it built its empire.

Annexation often causes controversy because the people living in the annexed territory might not want to belong to the conquering nation. Sometimes annexation happens peacefully when regions vote to join a country, but other times it happens against the wishes of the people who live there.

As a noun, an annex also means an additional building attached to a main building, like a school annex that provides extra classroom space.