Conquest

Conquest involves achieving control of another entity's territory through war or coercion. Historically, conquests occurred frequently in the international system, and there were limited normative or legal prohibitions against conquest.

The onset and diffusion of nationalism (the belief that nation and state should be congruent), especially in the 19th century, made the idea of conquest increasingly unacceptable to popular opinion. Prohibitions against conquest were codified with the establishment of the League of Nations following World War I and of the United Nations at the end of World War II.

Scholars have debated the strength of a norm against conquest since 1945. Conquest of large swaths of territory has been rare since the end of World War II. However, states have continued to pursue annexation of small territories. Provided by Wikipedia