After the end of First World War, it was realized that how horrible & destructive this was this war for the mankind & it’s coming generations. For securing the future from the devastation of the war, some big countries founded League of Nations nevertheless; it could not achieve the goal for which League of Nations was established.
In other words, despite the existence of League, the Second World War took placed which proved more destructive and dangerous for the common man on the world level. The atomic experiment was carried in this war which eliminated the two cities of Japan; Hiroshima and Nagasaki from the surface.
There was a few countries which acquired the membership of UNO but the number of members increased rapidly and today, United Nations Organization is the biggest institution of the world.