The armed forces of the United States are just over 200 years old and have been involved in at least 318 military operations, according the the House Committee on Foreign Affairs. That number doesn't count humanitarian missions or CIA operations.
So it may surprise you that America has only been at war 11 times.
Article I, Section 8 of the U.S. Constitution gives the power to declare war exclusively to the Congress, without describing exactly how that should be done. Congress figured it out by June 17, 1812 when it gave its first authorization for war, against Great Britain.
The only 11 Congressional declarations of war were:
- Great Britain, 1812
- Mexico, 1846
- Spain, 1898
- Germany, 1917
- Austria-Hungary, 1917
- Japan, 1941
- Germany, 1941
- Italy, 1941
- Bulgaria, 1942
- Hungary, 1942
- Romania, 1942
That's not to say all the other engagements were illegal or a violation of the Constitution. There were other times the Congress authorized funds for military actions and later, to support United Nations Security Council resolutions. It just means those other engagements weren't officially a "war."
There's a distinct difference between a war declaration and an authorization of military force. The most important is that an official state of war triggers a new set of domestic laws, like giving the President the power to take over businesses and transportation systems, detaining foreign nationals, warrantless domestic spying, and the power to use natural resources on public lands. The authorization of force doesn't give the President these powers.
The current way the President as Commander-In-Chief uses the military is defined by the War Powers Resolution of 1973. It requires the President to notify Congress within 48 hours of committing armed forces to military action and forbids them from remaining for more than 60 days, with a further 30-day withdrawal period, without an authorization of the use of military force or a declaration of war. Congress passed the War Powers Resolution to reign in President Nixon's use of the military, even overriding his veto to pass the law.