Why was the League of Nations unable to stop the expansion of Japan, Italy and Germany?

The League had no actual power to punish the nations. All nations that signed onto the league had to ban war, but when Japan and Italy wanted to expand they simply left the league when it tried to stop them.

The League depended on the Great Powers to enforce its rulings (mostly Britain and France) both were reluctant to use military force. They economic sanctions levied by the League also failed since the affected nations could simply trade with nonmembers.