After the Second World War, the charter of the newly formed United Nations established an eleven-member Security Council and charged it with taking collective action in response to threats to world peace. The charter further provided that the five nations that were then the major powers would permanently have sole authority to cast vetoes. The reason given for this arrangement was that the burden of maintaining world peace would rest on the world' s major powers, and no nation should be required to assume the burden of enforcing a decision it found repugnant.
The statement 'no nation that was not among the major powers at the end of the Second World War would become a major power' is the reason given for the structure of the Security Council.