The original basis of the UN was the coalition of forces to defeat Imperial Japan and Nazi Germany. The post-war reconstruction efforts and development of international institutions developed later. To a great extent those on the security council were the key players in the Allies during World War Two, plus the Republic of China, who whilst not a particularly effective military force or coherent polity during the war, had the confidence of Roosevelt as a key international player.

From this beginning, it is quite possible to see how the United Nations as a military coalition, and dominated by the US and USSR in the early postwar de-colonisation era expanded from the few dozen original members, as new states gained sovereignty, to the nearly 200 members of today, whilst having to accept the de facto pre-eminence of the founding states through their 'seniority' and relative power.

This does not mean this situation is inevitable in the future, and changes have occurred such as the transfer of the permanent seat from the ROC to the PRC in 1971.

Though it falls outside the remit of AskHistorians, there is a current pending motion to expand the permanent membership of the security council to include Brazil, India, Germany and Japan, but is currently held up by China's veto powers over the inclusion of India and Japan simultaneously as a new permanent member, and the more generally slow ratification process.