Gain in territory by military conquest.
From the middle of the 19th century, the USA denied the admissibility of gain conquest and was joined in 1856 by Latin American states in pledging not to recognize any territorial change amongst themselves for any reason. In 1931 the USA formulated the Stimson doctrine indicating that it would not recognize any political or territorial change made by force.
From the 1940s the USA denied the claim of the USSR that the Baltic republics had joined the USSR. The UK and France ignored the annexation of Austria by Germany in 1938 although this was specifically prohibited by agreements following World War I. China annexed Tibet in the 1950s. It was the action of Iraq in endeavouring to annex Kuwait that led to the Gulf War. With the collapse of Yugoslavia, Serbia has acted to extend its territory by the incorporation of Bosnia--Herzegovina.