There are several reasons for the League of Nations' ineffectiveness. First, the League was created without the support of the United States, which was the most powerful country in the world at the time. Second, the League lacked a strong military force to enforce its decisions. Third, the League was often divided by national interests, and its members were unwilling to make sacrifices to resolve conflicts.
As a result of these weaknesses, the League of Nations was unable to prevent World War II from starting in 1939. The war was even more destructive than World War 1, and it resulted in the deaths of millions of people.
After World War II, the United Nations was created to replace the League of Nations. The UN was given more power than the League of Nations, and it has been more successful in preventing war and resolving international conflicts. However, the UN is still limited by the national interests of its member states, and it is not always able to resolve conflicts effectively.