The formation of the United Nations was a response to

After World War II, countries wanted a stronger way to cooperate internationally and prevent another major war. The United Nations was established in 1945 to support peace and cooperation among nations, so the correct choice is “the post-war desire for international cooperation.” The Truman Doctrine, Marshall Plan, and NATO were later parts of the Cold War response to tensions with the Soviet Union.

AI-written and checked against the verified answer.