Franklin D. Roosevelt
The president who remade the American state during the Depression, led it through the Second World War, and died four months before victory.

No American has held the presidency as long and none is likely to again, because the amendment limiting presidents to two terms was passed in direct response to him. Franklin Roosevelt won four elections, governed for twelve years and thirty-nine days, and left behind an American state and an international order that both still recognizably exist.
The polio and the persona
He was born rich in the Hudson Valley, a distant cousin of Theodore Roosevelt, and his early career was conventional and undistinguished. In 1921, aged thirty-nine, he was paralyzed from the waist down by what was diagnosed as polio. He never walked unaided again, wore heavy steel braces, and arranged with the White House press corps that he would not be photographed in his wheelchair, a request that was almost universally honored. Most Americans who voted for him four times did not fully register that he could not stand up on his own.
The New Deal
He took office in March 1933 at the pit of the Depression, with a quarter of the workforce unemployed and the banking system closed. His first inaugural gave the country its line about fear itself, and the first hundred days gave it fifteen major laws: a bank holiday and deposit insurance, farm price supports, industrial codes, the Tennessee Valley Authority, and public works.
The New Deal was not a coherent program. It was serial improvisation, some of it contradictory, and it did not end the Depression, which required war spending. What it did was change the terms of American politics permanently. After 1935 the federal government guaranteed bank deposits, paid old-age pensions through Social Security, protected the right to organize a union, and regulated the stock market. Before Roosevelt, none of that existed; after him, no administration of either party dismantled it.
The system pushed back. The Supreme Court struck down major New Deal laws, and in 1937 Roosevelt proposed adding justices to it. The plan failed, cost him political capital, and remains the standard cautionary tale about a president attacking the courts. He also built his majority on an alliance that included southern segregationists, which meant Social Security and labor law were written to exclude agricultural and domestic workers, and therefore most Black workers in the South.
The war
He saw the danger earlier than his country did and moved as fast as public opinion allowed, which was not very. Lend-Lease in March 1941 let him supply Britain without payment or a declaration. The arsenal of democracy phrase was his. After Pearl Harbor he made the strategic decision that decided the war's shape: Germany first, despite the attack having come from Japan.
He also authorized the internment of some 120,000 Japanese Americans, two-thirds of them citizens, an act with no military justification for which the United States formally apologized in 1988.
At Yalta in February 1945 he was visibly dying. He has been accused ever since of giving away eastern Europe there, which misreads the position: the Red Army already held it, and there was no instrument short of war for removing it.
What he left
He died of a cerebral hemorrhage on 12 April 1945, weeks before Germany surrendered, leaving a vice president who had been told nothing about the atomic bomb. His inheritance is double: a federal government responsible for economic security at home, and an American commitment to run the outside world, expressed in the institutions he pushed for at Bretton Woods and in the United Nations he named.