To join a new territory to an exsisting nation is called?

The term used for joining a new territory to an existing nation is "annexation." Annexation is the formal act of incorporating a territory into the existing territory of a country. It can occur through various means, such as conquest, purchase, or agreement between the parties involved. When a territory is annexed, it becomes part of the annexing country, and its residents usually acquire the citizenship and rights of citizens of the annexing country.