There are many reasons why the League of Nations failed. They are:
1.Not all major countries joined., e.g. The USA, Russia and Germany. The USA was a big and powerful nation who would be able to help the League of Nations in times of trouble.
2. The United States of America never became a member of the League of Nations
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3. Members preferred to look after their own interests. Britain and France were often unwilling to get involved in League affairs, preferring to make sure their national interest was defended.
4. It had no armed forces With member states unwilling to provide soldiers, the LON never had an army - its only weapon was sanctions.
5. It was unable to stop aggression by major powers Japan, then Italy were able to get away with invasions of other countries. This made the League appear powerless. This meant Germany could consider breaking the Treaty of Versailles.
6. Didn't make decisions quickly in many cases, making small issues much bigger than they should be.
7. Failed to bring about disarmament, so countries still had weapons, and would therefore be able to start another war.
8. The Depression made people worry about themselves, not about world peace, which was the League of Nations' main aim and point.
9. The League only managed to stop crises involving small and weak countires, not issues involving powerful nations such as Italy and Japan, in the Manchuria and Abyssinia Crises during the 1930s. This made the Crises even worse than they would have been if the League's members cooperated