The USA is the world's foremost economic and military power, with global interests and an unmatched global reach.
America's gross domestic product accounts for close to a quarter of the world total, and its military budget is reckoned to be almost as much as the rest of the world's defence spending put together.
The country is also a major source of entertainment: American TV, Hollywood films, jazz, blues, rock and rap music are primary ingredients in global popular culture.
The United States originated in a revolution which separated it from the British Crown. The constitution, drafted in 1787, established a federal system with a division of powers which has remained unchanged in form since its inception.
The US contains a highly diverse population, the product of numerous and sustained waves of immigration. Ethnic and racial diversity - the "melting pot" - is celebrated as a core element of the American ideology.
The 1964 Civil Rights Act outlawed racial and other discrimination, but race continues to be a live issue.
The election of Barack Obama as the country's first African-American president in November 2008 marked a defining moment in the country's chequered history of race relations.
Decline of the natives
The original people of north America, who made up several distinct groups of native Americans, went into decline with the arrival of settlers and now constitute a minority of the population.
The early settlers came predominantly from the British Isles. Large numbers of black Africans were taken as slaves to work the plantations of the Americas, while millions of Europeans in search of political freedom and economic opportunity constituted a third stage of immigration.
Today, Asians from the Pacific rim and Hispanics from the southern Americas are among those seeking what their predecessors wanted - the promise of prosperity and freedom which remains one of the defining hallmarks of "the American dream".
Despite relative prosperity in recent years, the gap between rich and poor remains a major challenge. More than 30 million Americans live below the official poverty line, with a disproportionate percentage of these being African-Americans and Hispanics.
Furthermore, the global financial crisis of 2008 has left the US facing its most challenging set of economic circumstances since the Great Depression of the 1930s.
The terrorist attacks of September 11 2001 had a momentous impact as the country continued to re-define its role as the world's only superpower.
In October 2001 the US led a military campaign in Afghanistan that unseated the Taleban regime. However, the man who ordered the 9/11 attacks, Al-Qaeda leader Osama Bin Laden, survived until 2011, when killed in a US special forces operation in Pakistan.
In March 2003 Washington initiated military action in Iraq which led to the toppling of the Iraqi leader Saddam Hussein.
US foreign policy has often mixed the idealism of its "mission" to spread democracy with the pursuit of national self-interest.
Given America's leading role on the international stage, its foreign policy aims and actions are likely to remain the subject of heated debate and criticism, as well as praise.