Franklin Delano Roosevelt (January 30, 1882 – April 12, 1945), also known as FDR, was the 32nd president of the United States, serving from 1933 until his death in 1945. He is the longest-serving U.S. president, as well as the only one to have served more than two terms. His first two terms were centered on combating the Great Depression, while his third and fourth saw him shift his focus to U.S. involvement in World War II. A member of the Democratic Party, Roosevelt previously served in the New York State Senate from 1911 to 1913 and as the 44th governor of New York from 1929 to 1932.

Born into the prominent Delano and Roosevelt families in Hyde Park, New York, Roosevelt graduated from Harvard University with a Bachelor of Arts. He was first elected to the New York State Senate in 1911, and served this seat before becoming the assistant secretary of the Navy under President Woodrow Wilson, holding this position from 1913 to 1920. Roosevelt was the running mate of James M. Cox on the Democratic ticket in the 1920 presidential election, which Cox lost to Republican nominee Warren G. Harding. In 1921, he contracted a paralytic illness that permanently paralyzed his legs. However, he returned to the public office upon being elected governor of New York in 1928, partly through the encouragement of his wife Eleanor, and during his tenure as governor, he promoted programs to combat the Great Depression. After receiving the Democratic nomination, Roosevelt defeated President Herbert Hoover in a landside victory in the 1932 presidential election.

During his first 100 days as president, Roosevelt spearheaded unprecedented federal legislation and directed the federal government during most of the Great Depression, implementing the New Deal, building the New Deal coalition, and realigning American politics into the Fifth Party System. He created numerous programs to provide relief to the unemployed and farmers while seeking economic recovery with the National Recovery Administration and other programs. He also instituted major regulatory reforms related to finance, communications, and labor, and presided over the end of Prohibition. He was reelected in 1936, upon defeating Alf Landon in one of the largest landslide victories in American history. Roosevelt was unable to expand the Supreme Court in 1937, the same year the conservative coalition was formed to block the implementation of further New Deal programs and reforms. Major surviving programs and legislation implemented under Roosevelt include the Securities and Exchange Commission, the National Labor Relations Act, the Federal Deposit Insurance Corporation, and Social Security. In 1940, he ran successfully for reelection, before the official implementation of term limits.

Following the Japanese attack on Pearl Harbor on December 7, 1941, Roosevelt obtained a declaration of war on Japan. When in turn, Japan's Axis partners, Nazi Germany and Fascist Italy, declared war on the U.S. on December 11, 1941, he secured additional declarations of war from the United States Congress. He worked closely with other national leaders in leading the Allies against the Axis powers. Roosevelt supervised the mobilization of the American economy to support the war effort and implemented a Europe first strategy. He also initiated the development of the first atomic bomb and worked with the other Allied leaders to lay the groundwork for the United Nations and other post-war institutions, even coining the term "United Nations". Roosevelt won reelection in 1944, but died in 1945 after his physical health seriously and steadily declined during the war years. Since then, several of his actions have come under criticism, such as his ordering of the internment of Japanese Americans. Nonetheless, historical rankings consistently place him among the three greatest American presidents, and he is often considered an icon of American liberalism.