In August 1928 fifteen nations signed the Kellogg-Briand Pact in Paris, and most of the world's states soon joined them. They agreed to renounce war "as an instrument of national policy in their relations with one another." Eleven years later Europe was at war again. Explaining that collapse is the first task of any history of the Second World War. Here we pick up the threads that ran from the 1920s to 1939.

Depression and dictatorship

Germany's first democracy, the Weimar Republic, carried the stigma of defeat and of a treaty most Germans thought unjust. It survived the hyperinflation of 1923, but the Great Depression after 1929 threw millions out of work and pushed voters toward the extremes. Adolf Hitler's National Socialists became the largest party in the Reichstag in July 1932, though they never won a majority in a free election. Conservative politicians who believed they could control him persuaded President Hindenburg to make Hitler chancellor in January 1933. Within months the Enabling Act let his government make laws without parliament, rival parties were banned, and the first concentration camps held political opponents. Hitler's program joined revenge for 1918 with a racial worldview: Germans needed "living space" in the east, to be taken from peoples he called inferior, and Jews were cast as the hidden enemy behind every misfortune.

Germany was not alone. Benito Mussolini had ruled Italy since 1922 and dreamed of a new Roman empire. In Japan, army officers acted on their own in 1931, staging an explosion on a railway line as a pretext to seize Manchuria from China, where Japan set up a puppet state the next year. The League of Nations condemned the move, and Japan simply left the League.

Historians still argue about how much the Versailles treaty itself caused the war. Some, following the economist John Maynard Keynes, whose 1919 book attacked the treaty as vindictive and economically foolish, blame its harshness. Others reply that Germany paid only a fraction of the reparations demanded and that the treaty left Germany united and potentially the strongest power in Europe. On this view the trouble lay less in the treaty's terms than in the failure either to enforce them firmly or to revise them generously.

Tests the democracies failed

Each act of aggression tested whether anyone would resist. In 1935 Italy invaded Ethiopia, a fellow League member. The League imposed sanctions but left oil off the sanctions list, and Italy completed its conquest. In March 1936 German troops marched into the Rhineland, a border zone the Versailles and Locarno treaties had demilitarized; France, which still had the stronger army, did not move. From 1936 to 1939 Germany and Italy armed General Franco's side in the Spanish Civil War, and German bombers destroyed the town of Guernica in 1937, while Britain and France held to a policy of non-intervention. In March 1938 Germany annexed Austria.

Munich and the appeasement debate

Hitler next demanded the Sudetenland, the German-speaking border regions of Czechoslovakia. At Munich in September 1938, Neville Chamberlain of Britain and Edouard Daladier of France agreed with Hitler and Mussolini that Germany would take the Sudetenland. The Czechoslovaks were not even at the table. In March 1939 Hitler seized the rest of the Czech lands, showing that his aims went far beyond uniting Germans. Britain and France then guaranteed Poland's independence.

Appeasement, the policy of settling grievances through concessions to avoid war, has been a byword for cowardice since 1939. Its defenders deserve a fair hearing. Many Britons, remembering the slaughter of 1914 to 1918, believed another war would wreck civilization. Some thought Germany had real grievances from Versailles. The dominions, such as Canada and South Africa, were reluctant to fight over Czechoslovakia. Above all, revisionist historians who studied the cabinet papers argue that Britain needed time to rearm, and that the year after Munich gave the Royal Air Force the modern fighters and radar network that saved the country in 1940.

Critics answer that concessions rewarded aggression and made Hitler bolder. Munich cost the Allies Czechoslovakia's well-equipped army and its border fortifications, and Germany may have gained more from the extra year than Britain did. It also convinced Joseph Stalin that the Western powers could not be trusted. On 23 August 1939 Germany and the Soviet Union signed a non-aggression pact whose secret protocol divided Eastern Europe into spheres of influence, including a partition of Poland. Hitler now had a free hand.

Worked example: the view from Chamberlain's chair

Imagine you are a British minister in September 1938. Your military chiefs warn that the air defenses are not ready and that Britain cannot help Czechoslovakia directly. France is hesitant, and voters dread the bombing of cities. Hitler insists this is his last territorial demand. On these facts, signing at Munich looks reasonable. Now add what you could see only dimly: Hitler's record of broken promises since 1935, his open talk of conquest in the east, and the chance that the Czech army would have fought hard. The lesson historians draw is not simply "never negotiate." It is that a policy should be judged by the information available, by whether the other side's goals are limited, and by what happens if the bet is wrong. At Munich the bet rested on Hitler's word, and his word proved worthless within six months.