What was the major weakness of the league of nations?

Well for starters Germany, Russia and America weren't members. The League of Nations had no army so could not enforce its will other than by using trade sanctions. During the interwar years the world experienced economic depression and therefore no members wanted to stop trading with other countries. Any decision made by the league had to be unanimous so if any country disagreed the decision was invalid. The league didn't meet often enough to solve problems and Britain an France allowed italy to do what it wanted because they needed italy as an ally in a possible future war against Germany (WW2), therefore when Italy invaded Abyssinia (modern day Ethiopia) they did nothing. This showed the league to be week and other countries including germany did not obey its wishes. You may also want to investigate the Japanese invasion of China and th policy of appeasement to help with your question