International relations (1919–1939) covers the main interactions shaping world history in this era, known as the interwar period, with emphasis on diplomacy and economic relations. The coverage here follows the diplomatic history of World War I.

The important stages of interwar diplomacy and international relations included resolutions of wartime issues, such as reparations owed by Germany and boundaries; American involvement in European finances and disarmament projects; the expectations and failures of the League of Nations; the relationships of the new countries to the old; the distrustful relations between the Soviet Union and the capitalist world; peace and disarmament efforts; responses to the Great Depression starting in 1929; the collapse of world trade; the collapse of democratic regimes one by one; the growth of economic autarky; Japanese aggressiveness toward China; fascist diplomacy, including the aggressive moves by Fascist Italy and Nazi Germany; the Spanish Civil War.Norman Rich, Great Power Diplomacy since 1914 (2003) pp 70–248.

Other articles cover causes of World War II in 1938–1939. See Second Sino-Japanese War regarding Japan and China. See appeasement regarding Germany's expansionist moves toward the Rhineland, Austria, and Czechoslovakia, and the last, desperate stages of rearmament as another world war increasingly loomed.

Background

Peace and disarmament There were no great wars in the 1920s. There were a few small wars on the periphery that generally ended by 1922 and did not threaten to escalate. The exceptions included the Russian Civil War of 1917–1922, Polish–Soviet War of 1919–1921, the Greco-Turkish War of 1919–1922, and some civil wars, such as in Ireland. Instead, the ideals of peace is a theme that dominated the international agenda of all major nations in the 1920s. British Labour leader and Prime Minister Ramsay MacDonald was especially articulate:

"So soon as you aim at a guarantee by a body like the League of Nations you minimize and subordinate the military value of the pact and raise to a really effective standard the moral guarantees that flow from Conciliation, arbitration, impartial, and judicial judgment exercised by the body."

League of Nations The main institution intended to bring peace and stability and resolve disputes was the League of Nations, created in 1919. The League was weakened by the non-participation of the United States, Germany, and the Soviet Union, as well as (later) of Japan. It could not handle the refusal of major countries, especially Japan and Italy, to accept adverse decisions. Historians agree it was ineffective in major disputes.F. P. Walters, A History of the League of Nations (Oxford UP, 1965). Available for free on the site of the UN in Geneva library .C. L. Mowat, ed. The New Cambridge Modern History, Vol. 12: 1898–1945 (1968) pp 242–68.

A series of international crises strained the League to its limits, the earliest being the Japanese invasion of Manchuria. Japan was censured and quit the League.Ian Hill Nish, Japan's Struggle with Internationalism: Japan, China, and the League of Nations, 1931-3 (Routledge, 1993). This was soon followed by the Abyssinian Crisis of 1934–1936, in which Italy invaded Ethiopia (Abyssinia), one of the two independent African nations. The League tried to enforce economic sanctions upon Italy, but to no avail. The incident highlighted French and British weaknesses, exemplified by their reluctance to alienate Italy and lose it as a counterweight against Hitler's Germany. The limited actions taken by the Western powers pushed Mussolini's Italy towards alliance with Hitler's Germany.George W. Baer, "Sanctions and security: The League of Nations and the Italian–Ethiopian war, 1935–1936." International Organization 27#2 (1973): 165–179. The Abyssinian war showed the world how ineffective the League was at solving disputes. It played no role in dealing with the Spanish Civil War. The league also could not resolve other smaller conflicts involving major European nations, such as the Rif War between Spain and Morocco separatists.Pablo La Porte, "'Rien à ajouter': The League of Nations and the Rif War (1921—1926)," European History Quarterly (2011) 41#1 pp 66–87, online

Disarmament

The Washington Naval Conference, also known as the Washington Arms Conference or the Washington Disarmament Conference, was a military conference called by US President Warren G. Harding and held in Washington, under the Chairmanship of Secretary of State Charles Evans Hughes from 12 November 1921 to 6 February 1922. Conducted outside the auspice of the League of Nations, it was attended by nine nations: the United States, Japan, China, France, Great Britain, Italy, Belgium, Netherlands, and Portugal. The Soviet Union was not invited. The conference focused on resolving misunderstandings or conflicts regarding interests in the Pacific Ocean and East Asia. The main achievement was a series of naval disarmament postals agreed to by all the participants, which lasted for a decade. These resulted in three major treaties – Four-Power Treaty, Five-Power Treaty (the Washington Naval Treaty), the Nine-Power Treaty – and a number of smaller agreements.Thomas H. Buckley, The United States and the Washington Conference, 1921–1922 (U of Tennessee Press, 1970).Raymond G. O'Connor, "The" Yardstick" and Naval Disarmament in the 1920s." Mississippi Valley Historical Review 45.3 (1958): 441–463. in JSTOR

Britain now took the lead. The successful London Naval Treaty of 1930 continued the warship limitations among the major powers first set out in 1922. The treaties of 1922–30 preserved peace during the 1920s but were not renewed, as the world scene turned increasingly negative starting in 1931.B. J. C. McKercher, "The politics of naval arms limitation in Britain in the 1920s." Diplomacy and Statecraft 4#3 (1993): 35-59.

The peaceful spirit of Locarno

The seven international treaties negotiated at Locarno in 1925 by the major powers in Europe (not including the Soviet Union) significantly strengthened the legitimacy of Germany, paving the way to its return to the role of a major power and membership in the League of Nations in 1926, with a permanent seat on its council. The Locarno Treaties marked a dramatic improvement in the political climate of western Europe in 1924–1930. They promoted expectations for continued peaceful settlements, often called the "spirit of Locarno". This spirit was made concrete when Germany joined the League in 1926, and the withdrawal of Allied troops occupying Germany's Rhineland.

Historian Sally Marks says:

"Henceforth the spirit of Locarno would reign, substituting conciliation for enforcement as the basis for peace. Yet for some peace remained a desperate hope rather than an actuality. A few men knew that the spirit of Locarno was a fragile foundation on which to build a lasting peace."

Nazi Germany had no use for a peaceful spirit. It repudiated Locarno by sending troops into the demilitarized Rhineland on 7 March 1936.Gaynor Johnson, Locarno Revisited: European Diplomacy 1920–1929 (2004) excerpt and text search

Outlawing war

The Kellogg–Briand Pact of 1928 resulted from a proposal drafted by the United States and France that, in effect, outlawed war to resolve "disputes or conflicts of whatever nature or of whatever origin they may be, which may arise among them". Most nations readily signed up and used the occasion to promote the goal of peaceful foreign policies. The only issue was the actual formula of the renunciation of all wars – the French wanted the definition restricted to wars of aggression, while the Americans insisted it should include all kinds of warfare. Historian Harold Josephson notes that the Pact has been ridiculed for its moralism and legalism and lack of influence on foreign policy. He argues that it instead led to a more activist American foreign policy. Its central provisions renouncing the use of war, and promoting peaceful settlement of disputes and the use of collective force to prevent aggression, were incorporated into the UN Charter and other treaties. Although civil wars continued, wars between established states have been rare since 1945, with a few exceptions in the Middle East, Africa, and Asia.Harold Josephson, Diplomatic History (1979) 3#4 pp 377–390.

In the United States, much energetic grass-roots support came from women's organizations and Protestant churches.Robert Moats Miller, "The Attitudes of the Major Protestant Churches in America toward War and Peace, 1919–1929." Historian (1956) 19#1 pp 13–38. 26p. Argues that by 1917 the Protestant leaders saw the necessity of a war to end all war; They accepted the Treaty of Versailles and promoted the League of Nations. In the 1920s, there emerged and extensive disarmament and internationalist movement which peaked with their optimism over the Kellogg-Briand Pact of 1928.

Criminalizing poison gas Poison gas became the focus of a worldwide crusade in the 1920s. Poison gas did not win battles, and the generals did not want it. The soldiers hated it far more intensely than bullets or explosive shells. By 1918, chemical shells made up 35 percent of French ammunition supplies, 25 percent of British, and 20 percent of the American stock. The "Protocol for the Prohibition of the Use in War of Asphyxiating, Poisonous, or Other Gases and of Bacteriological Methods of Warfare" (Geneva Protocol) was issued in 1925, and was accepted as policy by all major countries.Tim Cook, "'Against God-Inspired Conscience': The Perception of Gas Warfare as a Weapon of Mass Destruction, 1915–1939." War & Society 18.1 (2000): 47-69.

Protecting power In the interwar period, the diplomatic role of an official protecting power was formalized in the Geneva Convention of 1929. Protecting powers were allowed to inspect prisoner of war camps, interview prisoners in private, communicate freely with prisoners, and supply books for the prison library. However, a suggestion by the International Committee of the Red Cross that it be made responsible for ensuring compliance with the treaty was rejected.Howard Levie, "Prisoners of War and the Protecting Power' American Journal of International Law 55#2 (1961) pp 374–397. doi=10.2307/2196124.

Europe

United Kingdom

The UK was a troubled giant that was less of a dominant diplomatic force in the 1920s than before. It often had to give way to the United States, which frequently exercised its financial superiority. The main themes of British foreign policy include a role at the Paris Peace Conference of 1919, where Prime Minister Lloyd George worked hard to moderate French demands for revenge. He was partly successful, but Britain repeatedly had to restrain the French regarding Germany.Andrew Barros, "Disarmament as a weapon: Anglo-French relations and the problems of enforcing German disarmament, 1919–28." Journal of Strategic Studies 29#2 (2006): 301–321. Britain was an active member of the new League of Nations, but repeated frustration was the main result.Peter J. Yearwood, Guarantee of Peace: The League of Nations in British Policy 1914–1925 (2009).Susan Pedersen, "Back to the League of Nations." American Historical Review 112.4 (2007): 1091–1117. in JSTOR