The League of Nations was created from the Treaty of Versailles after WWI. However, unfortunately no countries were willing to create the League of Nations. They joined and created the League because Wilson, the president of one the strongest countries U.S, said he wanted to create a better world. So, the countries in the League seemed lacking of eagerness. How effective was the League of Nations, then?
It was true that the League of Nations had contributed to world peace, in spite of the lack of eagerness. For example, they successfully stopped the conflict between Yugoslavia and Albania, and between Finland and Sweden. In addition, they arbitrated between Germany and Poland, Britain and Turkey, and Greece and Bulgaria. Without the League of Nations there were many wars between countries. So, the League of Nations contributed to the peace a lot.
And we can't ignore the League's contribution to improving the world condition.
For example, they dealt with 1,000,000 POWs, and they freed 200,000 slaves. In addition they cut down on leprosy and malaria, improving people's health. Furthermore, they restrained drugs and banned opium trade. This improved the world condition a lot and made world closer to today's one that has better heath care, no drug, and human rights. So, the League of Nations had good effects in certain aspects.
However there were many failure of League of Nations, too. For example, Corfu was a specific failure. This conflict was between Italy and Greece, and it was obvious that Italy should be blamed. But the League of Nations asked Greece to pay and apology to Italy, because Italy gave more money than Greece to the League. This shows that the League had not much power because they chose money instead of justice. So, League of Nations received bad...