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.
"Listen, we can do this thing if we promise not to tell Congress. So shut up."