According to the United States Constitution, if a sitting president wants to use the U.S. military against another nation, the office holder must first ask Congress for a specific declaration of war.
The United States has only actually declared war a handful of times. Since its ratification, the Constitutional function of war has only been used against Great Britain in 1812, Mexico in 1846, Spain in 1898, the Triple Alliance powers in World War I and the Axis Powers in World War II. But the U.S. has engaged in more than 102 conflicts since its inception.
The problem with a declaration of war (for Congress) is that a state of war gives the Office of the President incredible power under the Constitution. Instead of doing that, legislators would prefer to give the president limited power using an Authorization for the Use of Military Force (AUMF).
