f area, by 1790 its population was only 4 million. It grew rapidly, however, reaching 7.2 million in 1810, 32 million in 1860, 76 million in 1900, 132 million in 1940, and 321 million in 2015. Economic growth in terms of overall GDP was even greater. Compared to European powers, though, the nation's military strength was relatively limited in peacetime before 1940. Westward expansion was driven by a quest for inexpensive land for yeoman farmers and slave owners. The expansion of slavery was increasingly controversial and fueled political and constitutional battles, which were resolved by compromises. Slavery was abolished in all states north of the Mason–Dixon line by 1804, but the South continued to profit from the institution, mostly from the production of cotton. Republican Abraham Lincoln was elected president in 1860 on a platform of halting the expansion of slavery.

Seven Southern slave states rebelled and created the foundation of the Confederacy. Its attack of Fort Sumter against the Union forces there in 1861 started the Civil War. Defeat of the Confederates in 1865 led to the impoverishment of the South and the abolition of slavery. In the Reconstruction era following the war, legal and voting rights were extended to freed slaves. The national government emerged much stronger, and because of the Fourteenth Amendment in 1868, it gained explicit duty to protect individual rights. However, when white Democrats regained their power in the South in 1877, often by paramilitary suppression of voting, they passed Jim Crow laws to maintain white supremacy, as well as new disenfranchising state constitutions that prevented most African Americans and many Poor Whites from voting. This continued until the gains of the civil rights movement in the 1960s and the passage of federal legislation to enforce uniform constitutional rights for all citizens.

The United States became the world's leading industrial power at the turn of the 20th century, due to an outburst of entrepreneurship and industrialization in the Northeast and Midwest and the arrival of millions of immigrant workers and farmers from Europe. A national railroad network was completed and large-scale mines and factories were established. Mass dissatisfaction with corruption, inefficiency, and traditional politics stimulated the Progressive movement, from the 1890s to 1920s. This era led to many reforms, including the Sixteenth to Nineteenth constitutional amendments, which brought the federal income tax, direct election of Senators, prohibition, and women's suffrage. Initially neutral during World War I, the United States declared war on Germany in 1917 and funded the Allied victory the following year. Women obtained the right to vote in 1920, with Native Americans obtaining citizenship and the right to vote in 1924.

After a prosperous decade in the 1920s, the Wall Street Crash of 1929 marked the onset of the decade-long worldwide Great Depression. Democratic President Franklin D. Roosevelt ended the Republican dominance of the White House and implemented his New Deal programs, which included relief for the unemployed, support for farmers, Social Security and a minimum wage. The New Deal defined modern American liberalism. After the Japanese attack on Pearl Harbor in 1941, the United States entered World War II and financed the Allied war effort and helped defeat Nazi Germany in the European theater. Its involvement culminated in using newly-invented nuclear weapons on two Japanese cities to defeat Imperial Japan in the Pacific theater.

The United States and the Soviet Union emerged as rival superpowers in the aftermath of World War II. During the Cold War, the two countries confronted each other indirectly in the arms race, the Space Race, proxy wars, and propaganda campaigns. The goal of the United States in this was to stop the spread of communism. In the 1960s, in large part due to the strength of the civil rights movement, another wave of social reforms was enacted which enforced the constitutional rights of voting and freedom of movement to African Americans and other racial minorities. The Cold War ended when the Soviet Union was officially dissolved in 1991, leaving the United States as the world's only superpower.

After the Cold War, the United States's foreign policy has focused on modern conflicts in the Middle East. The beginning of the 21st century saw the September 11 attacks carried out by Al-Qaeda in 2001, which was later followed by wars in Afghanistan and Iraq. In 2007, the United States entered its worst economic crisis since the Great Depression, which was followed by slower-than-usual rates of economic growth during the early 2010s. Economic growth and unemployment rates recovered by the late 2010s, however these economic gains were erased in 2020 due to the COVID-19 pandemic