The United States of America, also known as the United States or USA, is a federal republic comprising 50 states, a federal district (Washington, D.C.), and several territories. Located primarily in North America, it is bordered by Canada to the north, Mexico to the south, the Atlantic Ocean to the east, and the Pacific Ocean to the west. The country was originally a collection of British colonies established along the Atlantic seaboard, declaring their independence in 1776 with the issuance of the Declaration of Independence. After a revolutionary war against Great Britain, the newly formed United States ratified its first governing document, the Articles of Confederation, in 1781. However, the Articles proved ineffective, leading to the drafting of ‣ in 1787, which was ratified in 1788 and went into effect in 1789.
The United States underwent rapid territorial expansion during the 19th century, acquiring lands through purchase, negotiation, and conflict. The issue of slavery became increasingly divisive, culminating in The U.S. Civil War (1861–1865), after which slavery was abolished and civil rights extended to former slaves during the Reconstruction Era. Throughout the 20th century, the U.S. emerged as a major global power, participating in both World Wars and becoming a founding member of the United Nations. It played a significant role in the Cold War, a geopolitical struggle against the Soviet Union and its allies, which ended in 1991. Domestically, the Civil Rights Movement of the 1950s and 1960s sought to end racial segregation and discrimination against African Americans, significantly influencing subsequent social and political reforms.
As of the early 21st century, the United States is the world's leading economic, political, and military power, with significant cultural influence extending globally. It has a diverse population of over 330 million people, making it the third most populous country in the world. The nation's mixed-market economy is fueled by abundant natural resources, a well-developed infrastructure, and high productivity. With a complex history and dynamic present, the United States remains a focal point in the study of democratic governance, human rights, global diplomacy, and economic development.