Actually, that is strictly speaking the only legal use of our armed forces, and Congress supposedly does have the power (if it should choose to exercise it) to cut off funding after thirty or sixty days (I forget which.)

Unfortunately, the U.S. congress has become too accustomed to passing blanket authorizations (such as the war on terror) while weaseling out of actual declarations of war. That was a problem in Vietnam, where after a decade Congress put their its foot down and passed a bill disallowing the use of the military in Southeast Asia. That went from one unwise extreme (Johnson and Nixen waging an undeclared war) to the other (making it impossible to even try to support our South Vietnam allies - not that Nixon would have been able to get anything through Congress in the last year or two of his presidency.)