Once half of the civilizations reach the Atomic Era (or one civilization reaches the Information Era), the World Congress becomes the United Nations and Diplomatic Victory becomes possible. To win Diplomatic Victory, a civilization must receive enough delegate support on a World Leader resolution.
Unlike other resolutions, the World Leader resolution cannot be proposed by a civilization. Instead, every other session of the Congress is dedicated to deciding it. During these sessions, which alternate with sessions of regular proposals, the World Leader resolution is automatically proposed to the Congress and no other proposals are made.
The amount of delegate support required to win depends on the number of civilizations and city-states in the game, and can be seen on the World Congress and Victory Progress screens. If no civilization receives enough support to win a World Leader resolution, the two civilizations that received the most support will permanently gain additional delegates.