So the UN managed to assemble a combined military force of many different nations to hold back the North Korean army. This was possible because of the activity in the council by its member states, and because of the change of rhetoric since the beginning of the 20th century.
The League of Nations was formed as a predecessor to the UN and partly NATO, but it was wildly ineffective. None of the member states were willing to commit military forces to the council to enforce peace and it was all mostly influenced by the isolationism of the US and the appeasement and neutrality policy of the Allies. So basically the UN is a revamped version of the League of Nations because it is more effective and it is forcing member states to participate while the League of Nations did not manage to do that. And it is important to note that the North Korean crisis was stopped because of the military intervention of the UN. While the League of Nations would have done basically nothing if they still existed at that point.
