The 20th century witnessed two great global wars that are commonly referred to as World War I (1914–18) and World War II (1939–45). The combined military and civilian casualties of these two wars exceeded 60 million people, with a resulting devastating impact upon Europe and parts of Africa and Asia. These events were the catalyst for the modern constraints on the use of force. Until the beginning of the 20th century, international law made little effort to regulate the resort to war. Many international legal scholars of that era conceded that war was a normal part of the conduct of international relations and that international law had little role to play in the decisions made by States to go to war. Two major developments changed legal thinking on these matters. First, the 1899 and 1907 Hague Peace Conferences reached agreement on 16 Conventions and accompanying Declarations, which placed limitations on how belligerent States conducted themselves during hostilities, including the weapons they could use. These Conventions and Declarations, often referred to as ‘Hague law’, were a key component in the early development of the modern law of armed conflict. Second, the creation of the League of Nations following the conclusion of World War I placed constraints on the resort to the use of force. The Covenant of the League of Nations, found within the 1919 Treaty of Versailles, sought in art 11 to make ‘any war or threat of war’ a matter of common concern to the members of the League. This development was soon followed by the 1928 General Treaty for the Renunciation of War as an Instrument of National Policy (Kellogg–Briand Pact), which clearly provided for the renunciation of the resort to the use of force. Article 1 provided:
The High Contracting Parties solemnly declare in the names of their respective peoples that they condemn recourse to war for the solution of international controversies, and renounce it as an instrument of national policy in their relations with one another.
While ultimately these developments in international law proved unable to prevent the outbreak of World War II, they did provide the foundation for key provisions of the Charter of the United Nations (UN Charter), of which art 2(4) is one of the most significant.