Encarta:

The Constitution of the United States gives Congress alone the authority to formally declare war. But in several past conflicts Congress has relinquished this authority to the president. In fact, Congress has not issued a formal declaration of war since World War II.

U.S. presidents after World War II have assumed most of the authority to send U.S. troops into battle. The Korean War (1950-1953), for example, was regarded by the U.S. government as a police action rather than as a war, and President Harry S. Truman never sought a declaration of war from Congress. And in 1964 Congress passed the Gulf of Tonkin Resolution, which effectively ceded to President Lyndon B. Johnson the ability to wage war against Vietnam. Congress passed a similar resolution on January 12, 1991, authorizing President George H. W. Bush to use force against Iraq in the Persian Gulf War.

There was no declaration of war for any of these:

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).

1950 Puerto Rico. Jayuya uprising crushed in Ponce Oscar Collazo and Griselio Torresola were sent to assassinate President Truman. Torresola was killed and Collazo was critically wounded in a shootout with capital police and Truman's bodyguards.

1961 Bay Of Pigs Cuba. Ustrasined troops invade Cuba to overthrow Castro. Kennedy withholds promised air support.

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.

1966 Guatemala. Green Berets intervene against rebels.

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.

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.

1986 Raid on Tripoli Libya following a bomb attack on a West Berlin discotheque.

1989 President Bush sends troops to Panama to depose and capture Manuel Noriega, who had been indicted for drug trafficking.

1991 Persian Gulf War—the U.S. leads a coalition of 32 countries to drive Iraq out of Kuwait, which it had invaded.

1993 Mogadishu 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 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.

1998 Afghanistan, Sudan: American cruise missiles pounded sites in Afghanistan and Sudan in retaliation for the bombings of U.S. embassies in Kenya and Tanzania.

!998 Iraq President Clinton said "adm (Hussein) must not be allowed to threaten his neighbors or the world with nuclear arms, poison gas or biological weapons. Operation Desert Fox, a strong, sustained series of attacks, will be carried out over several days by U.S. and British forces, "Earlier today I ordered America's armed forces to strike military and security targets in Iraq. They are joined by British forces," "Their mission is to attack Iraq's nuclear, chemical and biological weapons programs and its military capacity to threaten its neighbors," Clinton also stated that, while other countries also had weapons of mass destruction, Hussein is in a different category because he has used such weapons against his own people and against his neighbors. "I think you give the commander in chief the benefit of the doubt," said George W. Bush, governor of Texas, on August 20, 1998, the same day as the U.S. counterstrikes.

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.

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.