League of Nations
The first permanent organization built to prevent war through collective security; it failed the 1930s and became the model and warning for the United Nations.
The League of Nations was the first attempt in human history to build a permanent institution whose whole job was to stop wars before they started. It opened in 1920, headquartered in Geneva, and its founding promise was radical: that states would settle their quarrels by arbitration and, if one nation attacked another, the rest would answer together. This was collective security, the notion that aggression against one is aggression against all. The idea did not die with the League. It is the beating heart of the United Nations that replaced it, and of NATO's Article 5. The League simply proved how hard the idea is to make real.
The design and its flaws
The machinery looked reasonable and worked poorly. An Assembly of all members and a smaller Council of great powers could recommend action, but nearly every important decision required unanimity, which handed a veto to anyone with an interest in doing nothing. The League had no army of its own; its ultimate weapon was economic sanctions, and even those depended on members choosing to enforce them. Worst of all was the hole at the center. Woodrow Wilson, the American president who had conceived the League, could not get his own Senate to ratify membership, so the world's rising power never joined. Germany was admitted only in 1926 and walked out in 1933; the Soviet Union joined in 1934 and was expelled in 1939. A club meant to police the great powers was missing several of them at any given moment.
The decade that broke it
Through the 1920s the League notched real if modest wins: it resettled refugees, arbitrated a dispute between Sweden and Finland over the Aland Islands, fought epidemics, and curbed human trafficking. Then the 1930s arrived and exposed the machinery as toothless. In 1931 Japan seized Manchuria; the League commissioned a report, mildly condemned the invasion, and watched Japan walk out in 1933. In 1935 Mussolini's Italy invaded Ethiopia; the League imposed sanctions so full of holes that they left out oil, while Britain and France were caught scheming behind its back to hand Mussolini much of what he wanted. Hitler, meanwhile, remilitarized, rearmed, and annexed at will as the League said nothing that mattered. Collective security had revealed its fatal condition: it is worthless without the will of great powers to back it, and that will was nowhere to be found.
What its ghost built
By the time the League was formally dissolved in April 1946, the war it had failed to prevent was already over and its successor was standing. The men who wrote the United Nations Charter in 1945 studied the League's autopsy carefully. They gave the new body a Security Council whose five great powers held permanent seats and vetoes, a design meant to keep the strong inside the tent rather than sulking outside it, as the United States had. The UN inherited the League's dream and its buildings in Geneva. It also inherited the hard lesson the League paid for in blood: a peace organization is only ever as strong as the powers willing to fight for it.