In 1941, the United States formally entered the Second World War as a member of the Allied powers after the surprise attack on Pearl Harbor. Its armed forces fought simultaneously in two military theaters, Western Europe and East Asia. The country also experienced an unparalleled transformation that saw the rapid expansion of its military, scientific, and industrial might. It also pursued the Manhattan Project, a top secret effort to develop the atomic bomb. The United States came out of the war as a superpower and the only nation with nuclear weapons. It played a significant role in establishing the United Nations and drafting the 1947 Constitution of Japan, which had been defeated in 1945.
Following the Second World War, the United States and the Soviet Union faced growing tensions that escalated into the Cold War, which would play out for the duration of the twentieth century. The Cold War also posed a potential threat of nuclear conflict, when in 1949 the Soviet Union detonated its first atomic weapon. The United States countered Soviet allies in the Korean War (a UN mandate) and the Vietnam War (without UN support) but avoided direct military conflict with the Soviet Union itself. The Space Race, another measure of U.S.–Soviet competition during the Cold War, culminated in the Apollo 11 mission that saw the United States land the first man on the Moon. The dissolution of the Soviet Union in 1991 ended the Cold War, leaving the United States as the world's sole superpower for the next two decades. In the 21st century, the United States has increasingly been challenged by China as a dominant world power.
The first documentary evidence of the phrase "United States of America" dates from a January 2, 1776 letter written by Stephen Moylan to George Washington's aide-de-campJoseph Reed. Moylan expressed his wish to go "with full and ample powers from the United States of America to Spain" to seek assistance in the revolutionary war effort. The first known publication of the phrase "United States of America" was in an anonymous essay in The Virginia Gazette newspaper in Williamsburg, Virginia, on April 6, 1776.
The second draft of the Articles of Confederation, prepared by John Dickinson and completed no later than June 17, 1776, declared "The name of this Confederation shall be the 'United States of America'." The final version of the Articles, sent to the states for ratification in late 1777, stated that "The Stile of this Confederacy shall be 'The United States of America'." In June 1776, Thomas Jefferson wrote the phrase "UNITED STATES OF AMERICA" in all capitalized letters in the headline of his "original Rough draught" of the Declaration of Independence. This draft of the document did not surface until June 21, 1776, and it is unclear whether it was written before or after Dickinson used the term in his June 17 draft of the Articles of Confederation.
The phrase "United States" was originally plural in American usage. It described a collection of states—e.g., "the United States are." The singular form became popular after the end of the Civil War and is now standard usage in the U.S. A citizen of the United States is an "American". "United States", "American" and "U.S." refer to the country adjectivally ("American values", "U.S. forces"). In English, the word "American" rarely refers to topics or subjects not directly connected with the United States.
Indigenous peoples and pre-Columbian history
It has been generally accepted that the first inhabitants of North America migrated from Siberia by way of the Bering land bridge and arrived at least 12,000 years ago; however, some evidence suggests an even earlier date of arrival. The Clovis culture, which appeared around 11,000 BC, is believed to represent the first wave of human settlement of the Americas. This was likely the first of three major waves of migration into North America; later waves brought the ancestors of present-day Athabaskans, Aleuts, and Eskimos.
Over time, indigenous cultures in North America grew increasingly complex, and some, such as the pre-Columbian Mississippian culture in the southeast, developed advanced agriculture, architecture, and complex societies. The city-state of Cahokia is the largest, most complex pre-Columbian archaeological site in the modern-day United States. In the Four Corners region, Ancestral Puebloan culture developed from centuries of agricultural experimentation. The Haudenosaunee, located in the southern Great Lakes region, was established at some point between the twelfth and fifteenth centuries. Most prominent along the Atlantic coast were the Algonquian tribes, who practiced hunting and trapping, along with limited cultivation.
In the early days of colonization, many European settlers were subject to food shortages, disease, and attacks from Native Americans. Native Americans were also often at war with neighboring tribes and European settlers. In many cases, however, the natives and settlers came to depend on one another. Settlers traded for food and animal pelts; natives for guns, tools and other European goods. Natives taught many settlers to cultivate corn, beans, and other foodstuffs. European missionaries and others felt it was important to "civilize" the Native Americans and urged them to adopt European agricultural practices and lifestyles. However, with the increased European colonization of North America, the Native Americans were displaced and often killed. The native population of America declined after European arrival for various reasons, primarily diseases such as smallpox and measles.
European settlers also began trafficking of African slaves into Colonial America via the transatlantic slave trade. Because of a lower prevalence of tropical diseases and better treatment, slaves had a much higher life expectancy in North America than in South America, leading to a rapid increase in their numbers. Colonial society was largely divided over the religious and moral implications of slavery, and several colonies passed acts both against and in favor of the practice. However, by the turn of the 18th century, African slaves had supplanted European indentured servants as cash crop labor, especially in the American South.
During the Seven Years' War (1756–1763), known in the U.S. as the French and Indian War, British forces captured Canada from the French. With the creation of the Province of Quebec, Canada's francophone population would remain isolated from the English-speaking colonial dependencies of Nova Scotia, Newfoundland and the Thirteen Colonies. Excluding the Native Americans who lived there, the Thirteen Colonies had a population of over 2.1 million in 1770, about a third that of Britain. Despite continuing new arrivals, the rate of natural increase was such that by the 1770s only a small minority of Americans had been born overseas. The colonies' distance from Britain had allowed the development of self-government, but their unprecedented success motivated British monarchs to periodically seek to reassert royal authority.
Although the federal government outlawed American participation in the Atlantic slave trade in 1807, after 1820, cultivation of the highly profitable cotton crop exploded in the Deep South, and along with it, the slave population. The Second Great Awakening, especially in the period 1800–1840, converted millions to evangelical Protestantism. In the North, it energized multiple social reform movements, including abolitionism; in the South, Methodists and Baptists proselytized among slave populations.
The California Gold Rush of 1848–1849 spurred migration to the Pacific coast, which led to the California Genocide and the creation of additional western states. The giving away of vast quantities of land to white European settlers as part of the Homestead Acts, nearly 10% of the total area of the United States, and to private railroad companies and colleges as part of land grants spurred economic development. After the Civil War, new transcontinental railways made relocation easier for settlers, expanded internal trade, and increased conflicts with Native Americans. In 1869, a new Peace Policy nominally promised to protect Native Americans from abuses, avoid further war, and secure their eventual U.S. citizenship. Nonetheless, large-scale conflicts continued throughout the West into the 1900s.
Civil War and Reconstruction era
Irreconcilable sectional conflict regarding the enslavement of Africans and African Americans ultimately led to the American Civil War. With the 1860 election of RepublicanAbraham Lincoln, conventions in thirteen slave states declared secession and formed the Confederate States of America (the "South" or the "Confederacy"), while the federal government (the "Union") maintained that secession was illegal. In order to bring about this secession, military action was initiated by the secessionists, and the Union responded in kind. The ensuing war would become the deadliest military conflict in American history, resulting in the deaths of approximately 618,000 soldiers as well as many civilians. The Union initially simply fought to keep the country united. Nevertheless, as casualties mounted after 1863 and Lincoln delivered his Emancipation Proclamation, the main purpose of the war from the Union's viewpoint became the abolition of slavery. Indeed, when the Union ultimately won the war in April 1865, each of the states in the defeated South was required to ratify the Thirteenth Amendment, which prohibited slavery except as penal labor. Two other amendments were also ratified, ensuring citizenship for blacks and, at least in theory, voting rights for them as well.
Reconstruction began in earnest following the war. While President Lincoln attempted to foster friendship and forgiveness between the Union and the former Confederacy, his assassination on April 14, 1865 drove a wedge between North and South again. Republicans in the federal government made it their goal to oversee the rebuilding of the South and to ensure the rights of African Americans. They persisted until the Compromise of 1877 when the Republicans agreed to cease protecting the rights of African Americans in the South in order for Democrats to concede the presidential election of 1876.
After World War II, the United States and the Soviet Union competed for power, influence, and prestige during what became known as the Cold War, driven by an ideological divide between capitalism and communism. They dominated the military affairs of Europe, with the U.S. and its NATO allies on one side and the Soviet Union and its Warsaw Pact allies on the other. The U.S. developed a policy of containment towards the expansion of communist influence. While the U.S. and Soviet Union engaged in proxy wars and developed powerful nuclear arsenals, the two countries avoided direct military conflict.