The prominent historical figure instrumental in the establishment of the United Nations was Franklin D. Roosevelt. As U.S. President during World War II, he helped conceptualize the organization's creation to promote international cooperation and prevent future conflicts. Roosevelt's vision for a global entity that fostered dialogue and mutual understanding laid the groundwork for the UN's formation in 1945. His leadership emphasized the importance of collective security, human rights, and peacekeeping, principles that remain central to the UN's mission today.