When the first U.S. Army troops landed in war-ravaged Tokyo early in September 1945, they were commanded by scores of young officers, many born just before World War I. These confident commanders, ex-civilians still in their thirties, had been part of a vast wartime expansion of American power to almost every corner of the globe. But their memories stretched back to the grim Depression years, to high school in the 1920s, and to childhood memories of World War I itself. This generation experienced a transformation in American life that was as dramatic and fundamental as any since the founding of the Republic. When they attended elementary school, the nation’s social and political rhythms were still attuned to those of a nineteenth-century agrarian world; by the time they learned that B-29 bombers had dropped atomic bombs on Hiroshima and Nagasaki, U.S. diplomacy, politics, and society had far more in common with the end of the twentieth century than with its start.

In just one-third of a century, from 1914 to 1945, Americans fought two world wars, survived the Great Depression, and saw the emergence of the most powerful trade union movement in the nation’s history. As war and revolution convulsed countries around the globe, politics became saturated with debate over the great “isms” of the twentieth century: imperialism, capitalism, socialism, fascism, and communism. The federal government became far more powerful but also more intimately involved with the lives of ordinary Americans. Women got the vote, African Americans demanded their right to vote, and immigrant Americans began to exercise the franchise in huge numbers. New modes of communication, consumption, and transport—movies, radio, chain stores, buses and cars—recast the texture of everyday life.

Four great developments transformed the United States and its place in the world during this third of a century. First, the United States became the world’s greatest power, in terms both of its economic infrastructure and its military influence overseas. In 1914, the United States was already the productive equal of Great Britain and Germany; but whereas both world wars sapped the economic vitality of the European combatants—and decimated an entire generation of young men—the American republic emerged from each conflict with ever greater economic and military strength. U.S. industry perfected mass production, put an automobile in the driveway of most families, and developed the kind of technologies—such as aviation, radar, telecommunications, and nuclear fission—that paid off in time of war. Early in 1941, Life magazine publisher Henry Luce proclaimed this era “the American Century.” Indeed, by the end of World War II, the relative power of the United States recalled the strength of Great Britain at the high noon of its Victorian empire or Rome in the reign of the Caesars.

Second, the federal government became enormously more powerful during these three short decades. In 1914, the post office was perhaps the only agency of the national government that touched the lives of ordinary Americans. But war and depression soon changed all that. U.S. participation in World War I proved a dress rehearsal not just for the next world conflict, but also for the transformative, intrusive impact of the New Deal fifteen years later. During the First World War, the Wilson administration mobilized the American people as never before: government regulated business, took over the railroads, tested and trained millions of military recruits, censored the mail, and sold war bonds in a huge propaganda effort to gain public support for the battles overseas. In the 1920s, Republican presidents tempered the growth of federal power, but even in that more conservative decade, President Herbert Hoover, who had been a highly effective World War I administrator, argued for the kind of nation-state activism that would have surprised his nineteenth-century predecessors.

Many veterans of World War I, including Assistant Secretary of the Navy Franklin D. Roosevelt, would deploy this same kind of organizational energy and ambition when the nation faced its next great crisis, the Great Depression. The Roosevelt administration’s New Deal built dams and roads; subsidized millions of farmers; regulated corporations, banks, and the stock market; and established Social Security, which would eventually become the single largest, most expensive, and, for many years, most popular of all government programs. Then came World War II, when the federal government achieved stupendous size and power: it raised an army and navy of sixteen million; it mobilized the nation’s leading universities for military research; and it launched the industrialization of previously rural or residential areas, such as southern California, large sections of the agricultural South, and Long Island. By 1944, the military was spending more than 40 percent of the nation’s entire gross domestic product.

Third, these were the years during which the labor movement moved to the center ring of American politics and culture. This took place in two stages, during both of which rank-and-file activism and government policy combined to generate an explosive growth in union size and influence. During World War I, the Wilson administration repressed antiwar radicals, but to advance U.S. war aims, the government also linked victory over German autocracy to the spread of an industrial democracy at home. American workers were therefore emboldened to denounce their employers as “Prussian” managers and “Kaiserism” in industry, and they joined unions in a vast wave that crested during the nationwide strikes of 1919, the largest working-class upheaval in the history of the republic.

Fifteen years later, industrial unionism finally won the kind of political support and legislative backing that had been absent during the First World War. The 1935 Wagner Act encouraged trade unionism as a spur to economic recovery and a bulwark of American democracy. But no matter how favorable the law was to labor, the mass unionism of the 1930s required the courageous activism of thousands of women and men. Organizing the biggest industries of their time—automobiles, steel, rubber, and electrical products—they built the new Congress of Industrial Organizations (CIO) and put its power and values close to the heart of a New Deal impulse that stretched well into the post–World War II years.

Fourth, this era of war and depression generated a remarkable transformation in the character of the American people. Although shipping shortages and dangers during the First World War—and the immigration restriction laws that followed—stanched the great transatlantic migrations, the impact of that European diaspora would have a lasting effect on their new country. In the 1930s and 1940s, the sons and daughters of the prewar immigrant wave—the ambitious, assimilationist “second generation”—helped to reestablish the Democratic Party; gave new energy to the labor movement; and poured creativity into science, film, and music. Immigrant daughters figured prominently in the labor upsurge of the 1930s, but women of all backgrounds took advantage of new opportunities in industry and offices. They streamed into jobs that were vacated by men during the two world wars and began to stay in the workforce longer. In 1914, the typical woman worker was an unmarried teenager; after 1945, she was married and in her twenties or thirties.

Southerners both Black and white also poured into the industrial workplace. Fleeing a generation-long agricultural depression, many white southerners moved north during the 1920s and 1930s. But the wartime “Great Migration” of African Americans proved even more dramatic. Starting in the First World War and renewing itself in the Second, a huge exodus out of the rural South swelled the African American population of Chicago, New York, Baltimore, Washington, D.C., Detroit, and Oakland, California, moving the aspirations of this community toward the center of U.S. politics. In 1914, African Americans were voiceless, and hidden behind a veil of nearly unchallenged white supremacy. Thirty years later, a vigorous civil rights movement, powerfully reinforced by hundreds of thousands of Black factory workers, had begun to challenge the old racial order on almost every front.