Why did the United Nations declare Iraq's annexation of Kuwait invalid?

The United Nations declared Iraq’s annexation of Kuwait invalid because it violated the fundamental principles of international law regarding state sovereignty and the prohibition of aggressive war.12

Under the United Nations Charter, sovereign title to territory cannot be acquired through the use of force. While the traditional doctrine of conquest once allowed states to claim territory gained via military victory, this practice was outlawed in the 20th century. The international community, including the UN, has established that aggressive war is contrary to international law, and consequently, territorial acquisitions achieved through such means are denied legal recognition.21

By invading and attempting to annex Kuwait in 1990, Iraq sought to control its neighbor's oil wealth and gain strategic military advantages. The UN Security Council’s response—which included trade sanctions and the authorization of military force to liberate Kuwait—reinforced the ethical and legal principle that the sovereignty of a state is protected from violation by other states and that force cannot be used to extinguish the independence of a sovereign political entity.34512