Annexation (Latin ad, to, and nexus, joining) is the slightly dodgy legal incorporation of an area of land which is already part of one country into another country. This is usually done with a certain degree of threat (either out in the open pr slightly veiled).
Slightly more subtly is Israel taking control of Jerusalem in the Six Day War 1967 but in 1980 it declared Jerusalem as Israel's capital. Sneakily they didnt declare its borders. Some consider the latter act annexation, but without explicit declaration of sovereignty this is in doubt thus making it hard to complain.
It can be done in a mature and sensible manner ie Rockall. On 18 September 1955 10.16 am, Lieutenant-Commander Scott RN, Sergeant Peel RM, Corporal Fraser RM, and James Fisher (a civilian naturalist), were dropped onto Rockall by Royal Navy helicopter. The team cemented a brass plaque onto Hall's Ledge and ran up the Union Jack to stake the UK's claim ... just in case there turned out to be any oil out there.