Last time we declared war on anybody was back in 1941, right after Pearl Harbor. Now a days we call it "Authorization for the Use of Military Force" and apparently they last forever. They give the president authority to spend money, dispatch troops, order air strikes and other military action. The old fashioned declaration of war became moot when the war was over. Anyhow Congress just decided to cancel out some Authorizations to kick ass in the Middle East passed some twenty years ago and still being used today. Not a bad idea.
The president can use military force to defend the United States which covers the small stuff but probably does not go as far as doing regime change on disagreeable nations. As a matter of procedure, and a matter of political survival, the president wants to have Congress and the bulk of the nation behind any serious military operations. They should remember that we lost the Viet Nam war for lack of domestic political support.