Significant U.S. Conflicts

During the first phase of its history, the United States tried to isolate itself from foreign conflicts. In the 1900s, however, the United States participated in two world wars, became a global superpower, and eventually became a major proponent of world trade.

1890
1898 The United States wins the Spanish-American War and acquires foreign territoriesPuerto Rico, Guam, and The Philippines.
1900
1910
1914 World War I begins.
1917 The U.S. abandons neutrality to join the Allies in world War I.
1918 World War I ends.
1920
1920 The Senate rejects the Treaty of Versailles and American membership in the League of Nations.
1921 The U.S. declares the war with Germany to be at and end.
1930
1939 Germany invades Poland and World War II begins.
1940
1941 Japanese forces assail Pearl Harbor. The U.S. declares war on the Axis Powers and enters World War II.
1945 End of World War II. The U.S. is a founding member of the United Nations.
1947 The end of World War II marks the beginning of the Cold War. Fearing the spread of Communism in war-torn Western Europe, the United States inaugurates the Marshall Plan.
1949 The U.S. and 11 other nations sign the North Atlantic Treaty, forming NATO.
1950
1950 Communist North Korea invades South Korea. President Truman sends American troops to defend South Korea. The U.S. goes on to lead forces from 15 other nations in the Korean War (1950–1953).
1960
1964 The U.S. Senate passes the Gulf of Tonkin Resolution at the request of President Johnson. The Resolution approves U.S. military involvement in the Vietnam conflict.
1970
1973 President Nixon orders a halt to offensive operations in North Vietnam on Jan. 15 and representatives from all sides sign a peace pact, ending the longest war in U.S. history. The last American troops depart by March 1973.
1980
1983 President Reagan orders an invasion of Grenada to establish order on the island and eliminate the Cuban military presence there. A U.S. peace-keeping force remains until 1985.
1989 The Berlin Wall comes down, marking the end of the Cold War.
1989 President Bush sends troops to Panama to depose and capture Manuel Noriega, who had been indicted for drug trafficking.
1990
1991 Persian Gulf War—the U.S. leads a coalition of 32 countries to drive Iraq out of Kuwait, which it had invaded.
1993 U.S., Canada, and Mexico ratify the NAFTA free-trade agreement.
1993 Somalia A U.S.-led multinational force attempts to restore order to war-torn Somalia so that food can be delivered and distributed within the famine-stricken country.
1994 Haiti After Haiti's democratically elected president Jean-Bertrand Aristide is ousted in a coup in 1991, a U.S. invasion three years later restores him to power.
1994–1995 Bosnia During the Bosnian civil war, which begins shortly after the country declares independence in 1992, the U.S. launches air strikes on Bosnia to prevent ethnic cleansing. It becomes a part of NATO's peacekeeping force in the region.
1995 The U.S. is a founding member of the World Trade Organization (WTO), formed to monitor and promote world trade.
1999 Kosovo Yugoslavia's province of Kosovo erupts in war in the spring of 1999. A U.S.-led NATO force intervenes with air strikes after Slobodan Milosevic's Serbian forces uproot the population and embark on a plan of ethnic cleansing of Kosovo's ethnic Albanian population.
2000
2001 After the September 11, 2001, terrorist attacks, the U.S. declares war on terrorism.
2001 Operation Enduring Freedom—the U.S. invades Afghanistan and deposes the Taliban, who had sheltered terrorist leader Osama bin Laden.
2003 The U.S. launches Operation Iraqi Freedom, an invasion of Iraq, as part of the war on terrorism.
2010
2010 On Aug. 31, President Obama announces the end of U.S. combat missions in Iraq.
2010 Operation New Dawn begins on Sep. 1. A U.S. transitional troop force will remain in Iraq, tasked with the mission: "advising and assisting Iraq's Security Forces, supporting Iraqi troops in targeted counterterrorism missions, and protecting our civilians."