I remember when North Korea invaded South Korea. Harry S Truman was often a lucky man. In 1950 the world, most of it, at least the white and western portions, believed in the United Nations. And the Soviet Union sometimes boycotted sessions of the Security Council. The invasion happened during a boycott, so the Security Council was able to agree on the use of force to oppose it.
(For those many people who don't know the structure of the UN, almost all "nations" are included in the General Assembly (which in 1950 also included Ukraine plus another Soviet republic as well as the USSR) but the Security Council was supposed to be the fast-acting executive body with five permanent members (the WWII allies of USA, UK, France, (Nationalist) China, and USSR plus a rotation of other members. Each of the permanent members could veto action, which during the course of 72 years has eroded the UN's ability to act.)
So the Korean War was not the US and South Korea against North Korea and eventually Red China--it was the UN against the Reds. Wikipedia says 21 countries contributed troops, though the US provided the bulk of those coming from outside Korea.
So 72 years later we have a country invading another country, one of the permanent members of the Security Council, and neither Russia nor China is boycotting, so it's impossible for the Security Council to act. If it were possible, then NATO would have had cover to provide planes and troops to war. But as it is the UN becomes even more irrelevant.
I shed a tear for the dreams of the people after WWII who thought they'd fixed the problems of the League of Nations and the UN would lead the way to a better world.