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.
Freedom and prosperity
Statue of Liberty: First glimpse of the US for many past migrants
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.
The Freedom Tower will replace the World Trade Center
In October 2001 the US led a military campaign in Afghanistan which unseated the Taleban regime. However, the man thought to have inspired the 9/11 attacks, Al-Qaeda leader Osama Bin Laden escaped the operation and eluded the US 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.
Democratic Senator Barack Obama was elected the first black president of the United States in an historic election win in November 2008.
His emphatic victory over his opponent John McCain, who struggled in vain to distance himself from the unpopular presidency of George W. Bush, ended eight years of Republican rule in the White House.
Mr Obama clinched the Democratic Party's nomination in June 2008, following a long and bruising primary race with former first lady and fellow Democratic senator, Hillary Clinton.
He ran for president on a ticket promising change, and came to office riding a wave of high expectations from his supporters, both at home and abroad.
President Obama inherited a formidable in-tray of problems from Mr Bush. The US was facing its worst economic crisis since the Great Depression of the 1930s, and his administration the task of determining the next steps in the lingering conflicts in Iraq and Afghanistan.
He made reform of the healthcare system to extend coverage and reduce ballooning costs one of his top domestic priorities.
Despite a tortuous drafting process and vociferous Republican opposition, Mr Obama and Democrats in Congress finally succeeded in passing a health care bill in March 2010.
However, the health reform, along with the $787bn stimulus package passed in February 2010 to shore up an ailing economy, galvanised opposition among some Americans to Mr Obama's agenda.
The American Right in particular worries about what it sees as moves to extend the role of the state in the economy, and the threat of excessive public debt.
The rise of the conservative Tea Party movement in 2009 re-energised the Republicans and helped them to capitalise on popular discontent at the slow pace of America's economic recovery.
The Republicans made sweeping gains in mid-term elections in November 2010, regaining control of the House of Representatives.
The mid-term election result represented a serious setback for Mr Obama, as with his opponents controlling the House the president is likely to find it much harder to turn his plans into laws.
In the autumn of 2011 anti-capitalist protestors took to the streets of major cities, marching under the slogan "Occupy Wall Street", against "corporate greed" and increasing government debt. The protests inspired marches in other cities worldwide in October 2011.
In May 2011, he was widely applauded domestically - including by the Right - for his decision to order the operation that killed Al-Qaeda leader Osama Bin Laden.
Barack Obama is widely acknowledged as a charismatic figure and is noted for his stirring oratory. He was born in 1961 in Hawaii, the son of a black Kenyan father and a white American mother.
After attending an elite Hawaii academy and Columbia University in New York, he went on to Harvard Law School, where he graduated in 1991.
After graduating, Mr Obama practiced law and did community work in Chicago, where he also became active in the Democratic Party. He won a seat in the Illinois state senate in 1996, and followed this up by winning a US Senate seat in 2004.
The US has the most highly-developed mass media in the world. Its dramas, comedies, soap operas, animations, music videos and films have a global audience and are part of the staple fare of broadcasters worldwide.
CABLE NEWS CHANNELS
Fox News has been accused of promoting conservative views
TV is America's most popular medium. ABC, CBS and NBC ruled the roost for decades until the mass take-up of cable and satellite and the arrival of the Fox network. Fox News is the dominant US cable news network. Mainstream TV is slick, fast-moving and awash with advertising. Ratings and advertising revenues spell life or death for individual shows. The switchover to digital took place in June 2009.
There are around 10,000 commercial radio stations. In cities, there are services to satisfy almost every taste. News, sports and talk stations predominate on mediumwave (AM), with music on FM. Subscription satellite radio offers hundreds of channels and has attracted millions of customers.
Freedom of expression is guaranteed by the constitution, and some broadcast outlets give airtime to extreme hues of political - often right-wing - and religious thinking.
Public broadcasting is partly government-funded, but also supported by private grants. Universities and colleges operate outlets. National Public Radio - with more than 600 member stations - offers a more highbrow mix of news, debate and music without advertising. Public TV services operated by PBS have a mission to provide "quality" and educational programming.
The government sponsors TV and radio stations aimed at audiences outside the US. Lately, services for audiences in the former Soviet bloc have been cut, while stations targeting audiences in the Middle East and Asia have been launched.
There are more than 1,500 daily newspapers in the US, most of them with a local or regional readership. Hard-copy circulations are in decline as readers turn to the web.
The US is the home of the internet. Some 270 million Americans are online (InternetWorldStats.com, March 2011), comprising more than 78 per cent of the population. Seventy-four per cent of Americans use social networks and blogs, and 62 per cent are active on Facebook. (Nielsen, 2010).
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