The United States of America (USA or U.S.A.), commonly referred to as the United States (US or U.S.), America, and sometimes the States, is a federal republic consisting of 50 states and a federal district. The 48 contiguous states and Washington, D.C., are in central North America between Canada and Mexico. The state of Alaska is the northwestern part of North America and the state of Hawaii is an archipelago in the mid-Pacific. The country also has five populated and nine unpopulated territories in the Pacific and the Caribbean. At 3.71 million square miles (9.62 million km2) and with around 318 million people, the United States is the world's 3rd or 4th-largest country by total area and third-largest by population. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries. The geography and climate of the United States is also extremely diverse, and it is home to a wide variety of wildlife.
Paleo-Indians migrated from Eurasia to what is now the U.S. mainland around 15,000 years ago, The current Constitution was adopted on September 17, 1787. The first ten amendments, collectively named the Bill of Rights, were ratified in 1791 and designed to guarantee many fundamental civil rights and freedoms.
Driven by the doctrine of manifest destiny, the United States embarked on a vigorous expansion across North America throughout the 19th century. The American Civil War ended legal slavery in the country. By the end of the 19th century, the United States extended into the Pacific Ocean, and its economy began to soar. The Spanish–American War and confirmed the country's status as a global military power. The United States emerged from as a global superpower, the first country with nuclear weapons, and a permanent member of the United Nations Security Council. As part of the Cold War, American troops fought Communist forces in the Korean War of 1950–53 and fought a proxy war in Southeast Asia with the Vietnam War (1955-1975). The end of the Cold War and the dissolution of the Soviet Union left the United States as the sole superpower.
The United States is a developed country and has the world's largest national economy. The economy is fueled by an abundance of natural resources and high worker productivity. While the U.S. economy is considered post-industrial, it continues to be one of the world's largest manufacturers. The country accounts for 37% of global military spending, being the world's foremost economic and military power, a prominent political and cultural force, and a leader in scientific research and technological innovation.