President can, legally, use the military in combat operations (including sending Tomahawk missiles into an airfield in Syria) without congressional approval, or even letting them know he's going to do so in the first place
There has been a lot of discussion about whether or not the attack on an airfield by American Tomahawk missiles in early April of 2017 was a constitutional action by President Donald J. Trump, despite the fact that the military action occurred prior to the President receiving some kind of congressional approval for it.
First, let's set aside the "Congress has the authorization to declare war" argument, which argues the President cannot wage war without a declaration of war. While it is true that in Article I, Section 8 of the United States Constitution the power to "Declare War" is granted to the U.S. Congress, that is not the issue in question, here. There is no necessity for a declaration of war to accompany every military action. The manners and procedures of such a requirement would be burdensome and obstructive to the President's need to be decisive and swift in military matters, when a quick hand is called for. A Declaration of War is a formal, legislative act that informs the world of a state of war existing between the bodies named in that declaration, and also reveals that a long, drawn-out conflict is the subject of such a declaration. Could you imagine how long it would take to get Congress to agree on the conditions of the declaration for every little military operation? The fighting would be over before we could act, and our homeland or interests we wanted to protect would be in grave danger, as a result.
