The League of Nations and collective security
The Second World War began in Europe in September 1939 and became a global war in December 1941. Historians explain it by linking two kinds of cause. Structural causes were long-term conditions that made the world more dangerous. Short-term causes were the chain of events that turned that danger into fighting. The first structural cause is the failure of the body set up after 1919 to keep the peace.
The principle that members of an international organisation act together against any state that commits aggression, so that no aggressor can expect to get away with it.
The League of Nations never became the organisation President Woodrow Wilson had hoped for. It lacked the power to stop a determined great power using force, so it could not guarantee collective security. Some historians see its failure as a key cause of the Second World War.
The League's main enforcement tool was economic sanctions, meaning members would stop trading with an aggressor. Sanctions only work when members are willing to lose that trade themselves. As the next section shows, the Great Depression made them far less willing.
In 1931 the Japanese army conquered Manchuria in northern China. The League ruled against Japan and condemned the invasion, but the army simply carried on, and Japan left the League in 1933.
Condemnation without action showed other expansionist states that the League could be ignored.
A weak League did not start a war by itself. It removed a brake. The rest of the 1930s shows what happened when aggressive states tested whether anyone would apply that brake.
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