Annexation
Annexation is the legal merging of some territory into another body. A city might annex unincorporated areas or a country might annex other disputed territories. Sometimes also used to refer to mergers of countries.
One example of annexation was the United States of America's attempt to merge disputed parts of the Pacific Northwest of North America into their country. James Knox Polk, the 11th President of the United States, used as a successful campaign slogan "54-40 or fight!", commenting on the latitude of the western portion of the border between Canada and the United States, a border which was later officially set at the 49th parallel.