美国在苏联解体后成为了世界上唯一的超级大国。实力之强横，使美国在国际上有着超然无比的地位。美国这时在海外依旧进行着各种军事行动，举例1991年海湾战争美国就成功战胜伊拉克，并对伊拉克实行经济制裁。
1992年比尔·克林顿上台后，得益于数字革命和互联网带来的巨大机遇，美国经济迎来了其历史上持续时间最长的增长。反映科技股的纳斯达克指数一度升上历史高位，最后在2001年崩溃。
2001年9月11日，美国遭遇九一一事件，由奥萨马·本·拉登策划的恐怖活动袭击了纽约世界贸易中心和华盛顿五角大楼。作为回应，美国总统乔治·W·布什下令军队进入阿富汗并推翻了庇护本·拉登的极端势力塔利班政权。2003年，布什以反恐为名，推翻伊拉克独裁政府并抓捕了萨达姆·侯赛因。第二次伊战受到了大部分国家的赞同，但也有少数国家表示反对。
2000年美国总统选举是美国历史上结果最为接近的一次总统选举，也预示着政治两极化的到来。2005年8月，卡特里娜飓风引发的洪水严重摧毁了新奥尔良和密西西比州沿海地区，美国政府因灾前准备和灾后援救工作效率低下、动作缓慢而备受批评。
2008年的次贷风暴导致美国再次陷入经济衰退的危机之中, 同时由于次贷风暴造成的信用危机使美国多家大型金融从业机构破产或被政府接管,受此影响爆发2008年环球股灾,引发新一轮的全球性金融危机。
2009年1月20日巴拉克·奥巴马宣誓就任第44任美国总统，成为美国历史上首位非洲裔总统。
After the fall of the Soviet Union, the United States emerged as the world's sole remaining superpower and continued to involve itself in military action overseas, including the 1991 Gulf War. Following his election in 1992, President Bill Clinton oversaw unprecedented gains in securities values, a side effect of the digital revolution and new business opportunities created by the Internet . The 1990s saw one of the longest periods of economic expansion. Under Clinton an attempt to universalize health care, led by First Lady Hillary Rodham Clinton failed after almost two years of work on the controversial plan, however Hillary Rodham Clinton did succeed, along with a bipartisan coalition of members of congress, to establish the Children's Health Insurance Program.
The regime of Saddam Hussein in Iraq proved a continuing problem for the UN and Iraq's neighbors in its refusal to account for previously known stockpiles of chemical and biological weapons, its violations of UN resolutions, and its support for terrorism against Israel and other countries. After the 1991 Gulf War, the US, French, and British militaries began patrolling the Iraqi no-fly zones to protect Iraq's Kurdish minority and Shi’ite Arab population – both of which suffered attacks from the Hussein regime before and after the 1991 Gulf War – in Iraq's northern and southern regions, respectively. In the aftermath of Operation Desert Fox during December 1998, Iraq announced that it would no longer respect the no-fly zones and resumed its efforts in shooting down Allied aircraft.
During the 1990s the Al-Qaeda terrorist network and other Islamic fundamentalist groups attempted terrorist attacks against the United States and other nations. In 1993, Ramzi Yousef, a Kuwaiti national, and suspected Al-Qaeda operative, planted explosives in the underground garage of One World Trade Center and detonated them, killing six people and injuring thousands. Later that year in the Battle of Mogadishu, US Army Rangers engaged Somali militias supported by al-Qaeda in an extended firefight that cost the lives of 19 soldiers. President Clinton subsequently withdrew US combat forces from Somalia (there originally to support UN relief efforts). Terrorist attacks occurred in the 1996 Khobar Towers bombing in Saudi Arabia, and the 1998 United States embassy bombings in Tanzania and Kenya. There was an attempted bombing at Los Angeles International Airport and other attempts of acts of terrorism during the 2000 millennium attack plots. In Yemen the USS Cole was bombed in October 2000, which the government associated with Osama bin Laden's al-Qaeda terrorist network.
US responses to terrorist attacks included limited cruise missile strikes on Afghanistan and Sudan (August 1998), which failed to stop al-Qaeda's leaders and their Taliban supporters. Also in 1998, President Clinton signed the Iraq Liberation Act which called for regime change in Iraq on the basis of Saddam Hussein's possession of weapons of mass destruction, oppression of Iraqi citizens and attacks upon other Middle Eastern countries.
Al-Qaeda and other Islamic Fundamentalist groups were not the only groups responsible for terrorism during this time. In 1995, a domestic terrorist bombing took place at a federal building in Oklahoma City, which killed 168 people, and was the biggest terrorist attack on US soil since World War 2 at the time. It is believed that those responsible were associated with right-wing Christian groups.
In 1998, Clinton was impeached for charges of perjury and obstruction of justice that arose from lying about a sexual relationship with White House intern Monica Lewinsky. He was the second president to have been impeached. The House of Representatives voted 228 to 206 on December 19 to impeach Clinton, but on February 12, 1999, the Senate voted 55 to 45 to acquit Clinton of the charges.
The presidential election in 2000 between George W. Bush (R) and Al Gore (D) was one of the closest in the U.S. history, and helped lay the seeds for political polarization to come. Although Bush won the majority of electoral votes, Gore won the majority of the popular vote. In the days following Election Day, the state of Florida entered dispute over the counting of votes due to technical issues over certain Democratic votes in some counties. The Supreme Court case Bush v. Gore was decided on December 12, 2000, ending the recount with a 5–4 vote and certifying Bush as president.
At the beginning of the new millennium, the United States found itself attacked by Islamic terrorism, with the September 11, 2001 attacks in which 19 extremists hijacked four transcontinental airliners and intentionally crashed two of them into the twin towers of the World Trade Center and one into the Pentagon. The passengers on the fourth plane, United Airlines Flight 93, revolted causing the plane to crash into a field in Somerset County, Pennsylvania. According to the 9/11 Commission Report, that plane was intended to hit the US Capitol Building in Washington. The twin towers of the World Trade Center collapsed, destroying the entire complex. The United States soon found large amounts of evidence that suggested that a terrorist group, al-Qaeda, spearheaded by Osama bin Laden, was responsible for the attacks.
In response to the attacks, under the administration of President George W. Bush, the United States (with the military support of NATO and the political support of some of the international community) launched Operation Enduring Freedom which overthrew the Taliban regime which had protected and harbored bin Laden and al-Qaeda. With the support of large bipartisan majorities, the US Congress passed the Authorization for Use of Military Force Against Iraq Resolution of 2002. With a coalition of other countries including Britain, Spain, Australia, Japan and Poland, in March 2003 President Bush ordered an invasion of Iraq dubbed Operation Iraqi Freedom which led to the overthrow and capture of Saddam Hussein. Using the language of 1998 Iraq Liberation Act and the Clinton Administration, the reasons cited by the Bush administration for the invasion included the spreading of democracy, the elimination of weapons of mass destruction (a key demand of the UN as well, though later investigations found parts of the intelligence reports to be inaccurate) and the liberation of the Iraqi people. This second invasion fueled protest marches in many parts of the world.
In August 2005, Hurricane Katrina flooded parts of the city of New Orleans and heavily damaged other areas of the gulf coast, including major damage to the Mississippi coast. The preparation and the response of the government were criticized as ineffective and slow.
By 2006, rising prices saw Americans become increasingly conscious of the nation's dependence on supplies of petroleum for energy, with President Bush admitting a U.S. "addiction" to oil. The possibility of serious economic disruption, should conflict overseas or declining production interrupt the flow, could not be ignored, given the instability in the Middle East and other oil-producing regions of the world. Many proposals and pilot projects for replacement energy sources, from ethanol to wind power and solar power, received more capital funding and were pursued more seriously in the 2000s than in previous decades. The 2006 midterm elections saw Congresswoman Nancy Pelosi become Speaker of the United States House of Representatives and the highest ranking woman in the history of the U.S. government.
In addition to military efforts abroad, in the aftermath of 9/11 the Bush Administration increased domestic efforts to prevent future attacks. A new cabinet level agency called the United States Department of Homeland Security was created to lead and coordinate federal counterterrorism activities. The USA PATRIOT Act removed legal restrictions on information sharing between federal law enforcement and intelligence services and allowed for the investigation of suspected terrorists using means similar to those in place for other types of criminals. A new Terrorist Finance Tracking Program monitored the movements of terrorists' financial resources but was discontinued after being revealed by The New York Times. Telecommunication usage by known and suspected terrorists was studied through the NSA electronic surveillance program.
Since 9/11, Islamic extremists made various attempts to attack the US homeland, with varying levels of organization and skill. For example, in 2001 vigilant passengers aboard a transatlantic flight to Miami prevented Richard Reid (shoe bomber) from detonating an explosive device.
After months of brutal violence against Iraqi civilians by Sunni and Shi’ite terrorist groups and militias—including al-Qaeda in Iraq –- in January 2007 President Bush presented a new strategy for Operation Iraqi Freedom based upon Counter-insurgency theories and tactics developed by General David Petraeus. The Iraq War troop surge of 2007 was part of this "new way forward" and has been credited by some[who?] with a dramatic decrease in violence and an increase in political and communal reconciliation in Iraq.
As of 2009, debates continue over abortion, gun control, same-sex marriage, immigration reform, and the ongoing war in Iraq. Although the new Democratic Congressional majority promised to withdraw U.S. forces from Iraq, Congress continues to fund efforts in both Iraq and Afghanistan (however a withdrawal agreement has been agreed upon between the US and Iraqi governments). In the area of foreign policy, the U.S. maintains ongoing talks, led by United States Secretary of State Hillary Rodham Clinton, with North Korea over its nuclear weapons program, as well as with Israel and the Palestinian Authority over a two-state solution to the Israeli-Palestinian conflict; the Palestinian-Israeli talks began in 2007, an effort spearheaded by United States Secretary of State Condoleezza Rice. The George W. Bush administration also increased allegations implicating Iran, and more recently Syria, in the development of weapons of mass destruction.
In December 2007, the United States entered the longest post-World War II recession, which included a housing market correction, a subprime mortgage crisis, soaring oil prices, and a declining dollar value. In February 2008, 63,000 jobs were lost, a 5-year record for a single month. In September 2008, the crisis became much worse beginning with the government takeover of Fannie Mae and Freddie Mac followed by the collapse of Lehman Brothers. This economic crisis was considered the worst financial crisis since the Great Depression. By the end of 2008, the U.S. had lost a total of 2.6 million jobs.
In the presidential election of 2008, Senator Barack Obama, having narrowly defeated Senator Hillary Rodham Clinton for the Democratic nomination, ran on a platform of "Hope and Change". This, coupled with the economic crisis, helped aid his and running-mate Joe Biden's victory against the Republican ticket of Senator John McCain and Governor Sarah Palin. On November 4, Obama became the first African American to be elected President of the United States; he was sworn into office as the 44th President on January 20, 2009.
