anexionar

To annex — specifically for a state or power formally incorporating another territory into its own (anexionar un territorio). It's a political/historical term, distinct from the everyday anexar used for attaching documents or files.

• El imperio anexionó la región tras la guerra. — The empire annexed the region after the war.
• Varios países se opusieron a que anexionara la península. — Several countries opposed it annexing the peninsula.
• El tratado prohibía anexionar nuevos territorios sin acuerdo internacional. — The treaty prohibited annexing new territories without international agreement.

Confusing this with anexar (to attach a document) — anexionar is specifically about territorial or political annexation, a much narrower and more formal use.

Often confused with anexar.