The hope was that the League of Nations before World War II and the United Nations, its postwar successor, would provide a more effective way of ensuring world peace than the "Balance of Power" that Britain, in particular, had tried to maintain in Europe for centuries. This hope has not been fulfilled.
Although both organizations have contributed to keeping the peace in various ways, both have suffered from organizational weaknesses. The U.N. Security Council has often been powerless because of the veto that its five permanent members have used to prevent action that they dislike.
In practice, world peace still depends on old-fashioned balance-of-power politics, but the balance is constantly changing and political leaders need to be aware of these changes. This will be particularly true for the new Japanese government.