The United Nations was designed in 1945 to do what the League of Nations had failed to do — keep the great powers talking and prevent another world war. Its founders, meeting at Dumbarton Oaks and then San Francisco, included a deliberate flaw: a Security Council in which the five winners of the Second World War (the United States, the Soviet Union, the United Kingdom, France, and China — the Chinese seat held by the Republic of China until the People's Republic took it in 1971) each held a permanent veto. The flaw was load-bearing. Without it, none of the five would have joined. Roosevelt and Stalin alike had watched a powerless League collapse; the UN was built around the recognition that a body that excluded the great powers would be useless.
The veto has done exactly what its designers intended, and exactly what its critics predicted. It has prevented the UN from authorizing wars between the great powers — there has not been one since 1945, the longest such peace in modern history. It has also prevented the UN from acting on most of the genocides, civil wars, and atrocities of the past eighty years, because at least one P5 member has had a stake in protecting one of the parties. The Security Council's record is uneven where the veto happened not to bind: Korea (1950, authorized only because the Soviets were boycotting over the China seat), the first Gulf War (1990, when a collapsing USSR acquiesced), and a handful of African missions. Where a great power's interest was engaged, the record is abysmal — Rwanda in 1994, where 800,000 died while the Council dithered; Syria, where Russia and China cast more than a dozen vetoes; Yemen; and Ukraine, where the aggressor itself, Russia, sat as the Council's rotating president. Reform proposals — adding India, Brazil, Japan, Germany (the G4), and an African seat as permanent members — have been on the table since the 1990s and gone nowhere, because every existing P5 member benefits from keeping the club small, and any one of them can veto a Charter amendment that would dilute its own privilege.
The UN system's growing irrelevance to the most consequential conflicts of the current decade — Ukraine, Gaza, Sudan, the China-Taiwan question — is increasingly visible; the General Assembly passes resolutions the Council cannot enforce, and parties simply ignore them. Whether reform is possible, whether the system survives in current form, or whether something else gradually displaces it (BRICS-plus, regional bodies like the AU or NATO, ad-hoc coalitions of the willing) is one of the unsettled questions of the next decade, with consequences for whether any conflict still has a recognized place to be adjudicated.