The inability of the UN to take decisive action is a result of the fact that alliances have changed and geopolitical interests have shifted since the end of World War II.
Originally, the UN was not necessarily meant to be a global forum for all nations the way it is now. It was a club for the victors of World War II, and only nations who contributed to the war effort against the Axis were allowed to join. At that time, the United States, Britain, France, China, and the Soviet Union were allies, and they form the five nations with a veto power vote, but that alliance was short lived after the war, and has not gotten better since. They almost never agree on anything and thus nothing gets done.
You see, there’s an institution within the United Nations called the security council, which is a 15-nation body that acts on all of the executive motions of the United Nations. The 5 nations I mentioned previously have veto power, so if for instance someone wants UN forces to intervene to stop an invasion, all it takes is one of those 5 to stop it.
There are examples in history of when the UN was able to be an effective force, the main one being the Korean War. At that time, the USSR was boycotting the UN, and the other four powers were able to agree that North Korea was the aggressor, so UN troops fought in the Korean War against the DPRK and are a huge reason as to why South Korea was able to survive that war.
All in all, the ineffectiveness of the UN is at its core a result of the fact that it’s most powerful members never agree on how to solve problems, so nothing gets done and the UN is powerless to stop global conflict. It’s use is as a global forum now, not a guarantor of peace like it was intended.
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