Answer:
Franklin D. Roosevelt, also known as FDR, played a significant role in the United States' involvement and conduct of World War II.
Prior to the United States entering the war, Roosevelt provided support to the Allied powers through the Lend-Lease Act, which allowed the U.S to provide military aid to other countries without declaring war.
After the Japanese attack on Pearl Harbor in December 1941, Roosevelt declared war on Japan and subsequently Germany and the other Axis powers.
As President, Roosevelt held many war conferences with Allied leaders such as Churchill, Stalin and Chiang Kai-shek, where they developed strategies for the war and planned for the post-war world.
He also played a key role in the formation of the United Nations and pushed for the Atlantic Charter, which laid out war aims for the Allies and formed the basis of the UN Charter.
Roosevelt was also heavily involved in the conduct of the war, making decisions about which theaters to prioritize and approving major military operations such as D-Day and the Manhattan Project.
He also played a key role in maintaining public morale during the war through his famous "Fireside Chats" on the radio, where he would talk to the American people about the war effort and other important issues of the time.
Overall, FDR played a vital role in leading the United States into and through World War II, and his leadership and decisions had a significant impact on the outcome of the war.