The main weaknesses of the League of Nations were its lack of authority, limited membership, and post-war attitudes among member states. The League had no army to enforce its rules and depended on the voluntary cooperation of its members. Key states like Germany, the USSR, and the US were not original members. After World War I, most countries were unwilling or unable to commit militarily due to their weakened state and desire to avoid further conflict. These factors prevented the League from effectively responding to aggression like Japan's invasion of Manchuria in 1931 and Italy's invasion of Abyssinia in 1935.