At the beginning of the 20th century, New York was the most powerful state in the United States. Two-thirds of the nation’s leading corporations were headquartered in New York City, and the state as a whole produced one-sixth of the gross national product. Ex–New York governor Theodore Roosevelt had just succeeded President William McKinley—assassinated in Buffalo in 1898—to the White House.

The Progressive Era in politics had begun—to peak about a decade later, following the 1911 Triangle Shirtwaist factory fire that took the lives of 146 workers in Greenwich Village. Many progressive reforms were propelled through the state legislature by Democrat Al Smith, who later became one of New York’s greatest governors. A self-educated man, born poor on the Lower East Side, Smith helped create dozens of monumental labor, safety, education, and housing bills.

After World War I, the United States emerged as a world power, and nowhere was this newfound status more evident than in dazzling New York City. Business and manufacturing flourished. The Jazz Age arrived and the liquor flowed. F. Scott Fitzgerald came to town, and Jimmy Walker was elected mayor.

A dandified gentleman with a taste for the good life, Walker spent most of his time visiting nightclubs, sporting halls, and showgirls. Thanks to his late-night carousing, he rarely appeared at City Hall before 3 p.m., if at all. “No civilized man,” he once said, “goes to bed the same day he wakes up.”

The strict new federal immigration laws of 1921 and 1924 slowed the influx of foreigners into New York to a trickle, but Harlem boomed as black Southerners fleeing poverty took refuge in the city. In 1910, Manhattan was home to about 60,000 blacks; by 1930, that number had tripled. Harlem became the center for African American culture, with the Harlem Renaissance attracting writers and intellectuals such as Langston Hughes and W. E. B. DuBois, and jazz clubs and theaters attracting the likes of Duke Ellington and Chick Webb.

The Great Depression of 1929 hit New York State especially hard. By 1932 industrial production upstate had fallen by one-third, bread lines filled city blocks, and New York City banks shut down and reopened as soup kitchens. Scores of shantytowns called “Hoovervilles” dotted Central Park and other city parks upstate.

Enter Franklin Delano Roosevelt. In 1930, as governor of New York, he devised a five-point program to help the state cope with the economic disaster. In 1932, elected president of the United States, he applied and expanded his New York program into the national New Deal. New Yorkers went back to work on public works projects ranging from transportation to housing. Many were projects envisioned and developed by Robert Moses, the autocratic “master builder” largely responsible for the shape of modern-day New York.

However, like the rest of the nation, New York’s greatest boon to post-Depression recovery was World War II. Overnight the state’s factories and shipyards thrived anew, producing arms, uniforms, and other items for the war effort.

During the war, Columbia University at 116th Street and Broadway was the site of a nuclear experiment conducted by Dr. Robert Oppenheimer. Code-named the “Manhattan Project,” the experiment led to the creation of the world’s first atomic bomb, dropped on Japan in August 1945.