Right of conquest
History and arguments: Proponents state that the right of conquest acknowledges the status quo, and that denial of the right is meaningless unless one is able and willing to use military force to deny it. Further, the right was traditionally accepted because the conquering force, being by definition stronger than any lawfully entitled governance which it may have replaced, was, therefore, more likely to secure peace and stability for the people, and so the right of conquest legitimizes the conqueror towards that end.
The completion of colonial conquest of much of the world (see the Scramble for Africa), the devastation of World War I and World War II, and the alignment of both the United States and the Soviet Union with the principle of self-determination led to the abandonment of the right of conquest in formal international law. The 1928 Kellogg–Briand Pact, the post-1945 Nuremberg and Tokyo Trials, the UN Charter, and the UN role in decolonization saw the progressive dismantling of this principle. The UN Charter's guarantees the "territorial integrity" of member states, but enforcement difficulties in the 21st century lead to a contentious debate on possible re-emergence of the right of conquest as international law.

Conquest and military occupation: Until 1945, the disposition of territory acquired under the principle of conquest had to be conducted according to the existing laws of war. This meant that there had to be military occupation followed by a peace settlement, and there was no reasonable chance of the defeated sovereign regaining the land.  While a formal peace treaty "makes good any defects in title", it was not required.  Recognition by the losing party was not a requirement: "the right of acquisition vested by conquest did not depend on the consent of the dispossessed state". However, the alternative was annexation (part or in whole) which if protested as unlawful, a peace treaty was the only means to legitimize conquest in a time of war.  Essentially, conquest itself was a legal act of extinguishing the legal rights of other states without their consent.  Under this framework, it is notable that conquest and subsequent occupation outside of war were illegal.
In the post-World War II era, not all wars involving territorial acquisitions ended in a peace treaty. For example, the fighting in the Korean War paused with an armistice, without any peace treaty covering it. North Korea is still technically at war with South Korea and the United States as of 2024.

See also: Colonialism
Conquest
Debellatio
Discovery doctrine
Fait accompli
Franz Oppenheimer's "conquest theory" of the State
Imperialism
Invasion
Jungle justice
Just war theory
Mandate of Heaven
Manifest destiny
Might makes right
Prize (law)
Prize of war
Realism (international relations)
Revanchism
Responsibility to protect
Roerich Pact
Status quo ante bellum
Survival of the fittest
Uti possidetis
Vae victis
War of aggression
War trophy

References: 

Works cited: Korman, Sharon (1996). The Right of Conquest: The Acquisition of Territory by Force in International Law and Practice. Oxford University Press. ISBN 0-19-828007-6.