The United States of America (U.S.A.), commonly known as the United States (U.S.) or America, is a federal constitutional presidential republic located mainly in North America and composed of 56 states, three major self-governing districts, and various possessions. At over 3.8 million square miles (9.8 million km2) and with over 400 million people, the United States is the world's third-largest country by total land area and the fourth-most populous. The nation's capital is the city of Washington, and the largest city by population is Houston. Forty-nine of the states are contiguous and located in North America between Canada and Mexico. The state of Alaska is in the northwestern corner of North America, bordered by Canada to the east and by Russia across the Bering Strait to the west. The state of Hawaii is an archipelago located in the mid-Pacific Ocean, and the other four states, territories, and possessions are scattered across Oceania. The extremely diverse geography, climate, and wildlife of the United States make it a megadiverse nation.
Paleo-Indians migrated from Asia to North America by way of the Bering Land Bridge at least 15,000 years ago, and European colonization of the continent began in the late 16th century. The United States emerged from thirteen British colonies along the East Coast after numerous disputes between Great Britain and the colonies following the French and Indian War resulted in the American Revolution, which began in 1775 and ended in 1783 with the signing of the Treaty of Paris. The United States embarked on an expansion across North America during the 19th century, admitting new states and territories and displacing Native American tribes until it spanned the continent in 1848. During the late 19th century, the American Civil War led to the outlaw of slavery in the nation. By the end of the century, the United States had expanded into the Pacific and Caribbean, and its economy, driven largely by the Industrial Revolution, began to soar. The Spanish-American War and World War I confirmed the nation's status as a global military power, and it emerged from World War II as a global superpower. During the Cold War, a period of geopolitical tension between the United States and Soviet Union, the nations competed for influence and engaged in several technological competitions, culminating in the 1969 moon landing. The end of the Cold War and the collapse of the Soviet Union in 1991 left the United States as the world's sole superpower, a status that would eventually be disputed by nations such as China and India during the 21st century.