By design, the UN was not created for war-making purposes. Rather, it was intended (as was the League of Nations before it) to prevent wars through diplomatic negotiation. While there is a brief history of UN sanctioned wars (Korea, 1990-91 Gulf War), peacekeeping missions that interpose neutral troops between combatants is more common. But the peacekeeping depends on the politics and the politics, ultimately, depend on the Security Council. This, too, is by design. An international peace force is an idea, but not one whose time has come. There are too many concerns about both national sovereignty and fears of groups of nations ganging up against a single state to permit that to happen.
The question assumes a misinterpretation of the UN's role. It is not a "failed organization" by any manner of means. It is the most successful organization of its kind in the history of the world. The United Nations does not exist solely for the purpose of preventing wars; it does many, many, many other things as well -- promoting international cooperation on all sorts of fronts, and assisting with other problems. You might have heard of the UN High Commission on Refugees? Also, the UN was designed to be a place where countries could talk out inter-national conflicts and resolve them peacefully. Most of the world's conflicts, including those you allude to, are now intra-national, i.e. civil wars. A rebellious population within a country has no standing as a nation and so no seat in the UN, and the UN can't interfere with internal problems except at the request of the nation involved. The UN can send peacekeeping troops to a region if the host government asks for them, but they can't fight for one side; their function is to keep the two sides apart. There is in fact a peacekeeping mission in Syria right now, but its activities are limited to those that the Syrian government is prepared to let them take. They are there at its invitation. It is supervising the cease-fire and disengagement in the Golan Heights, not the civil war. Also, for political reasons, the UN's peacekeeping troops are never given real teeth. You don't see UN tanks or bombers. Nobody wants to support this idea for fear that it will be used on them.
Quora User, 22+ years as a Foreign Service Officer.
Two seconds of searching would have found the last three times this question was asked and the lengthy answers already pointing out the flaws in this line of reasoning. To summarize, the vast majority of what the UN does on a daily basis, and does pretty well, has nothing to do with international security. The problem with the UN SC has very little to do with the UN and everything to do with the US, China and Russia.