Nations that have resorted to the use of force since the Charter came into effect have typically invoked self-defense or the right of collective defense.
[citation needed] The UN Charter's guarantees the "territorial integrity" of member states, but enforcement difficulties in the 21st century[2] lead to a contentious debate on possible re-emergence of the right of conquest as international law.
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.
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".
[5] 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.