Where does it say the president needs a DOW to send the military into combat? John Adams and Jefferson both did. They didn’t understand the constitution, or what?
Try Article 1, Section 8, Clause 11. One could also attempt the case that Congress should be involved even for this, under Art. 1, Sec. 8, Clause 10 - I think we can both agree that employing chemical agents against a civilian population is an affront to (if not an outright violation of) the Law of Nations. But even that is likely more directed at instances involving American citizens, rather than those of another nation.
Look up Hamilton's commentary on Jefferson's 1807 address to Congress, regarding the Barbary Pirates for perspective on this issue.
When a belligerent party attacks its victim, a de facto ('by that act' for those in Rio Linda) state of war exists, and the victim need not declare so in order to retaliate. The Barbary Pirates having attacked American ships and enslaving American citizens and sailors thereby filled the role of the belligerent party, and Jefferson was therfore well within his powers to attack them (as agents of the Barbary States) without a Declaration of War.
As for others pointing to President Bush (43) and AFG/IQ, those states hosted belligerent non-state actors who had attacked us (or supported those who did). As allies/enablers, they are party to that attacks and therefore as liable to our retribution as Al-Qaeda itself. Taking out the enemy's support network and training areas (in concert with direct attacks on the enemy forces) is actually a very productive approach to eliminating an enemy. See Sun Tzu for thoughts on the indirect approach to warfare.
A Declaration of War is nice when someone attacks us, but not absolutely necessary to launch a military response to an attack. However, it does have the effect of resetting the national mindset and enabling the mobilization of the entire will and resources of the Nation toward the single goal of destroying the enemy. And that, IMNSHO, is where President Bush (43) missed a step.
And as we have seen over the past 8 years, a President will press his advantage so far as Congress allows him to do so. Congress chickened out with the AUMF for AFG/IQ, and completely ran away from BJCs use of cruise missiles and excursion into Serbia, and BHOs indiscriminate use of drones (particularly against an American citizen (al-Awaki) who should have either been brought to trial and stripped of his citizenship (that pesky Due Process thing), before termination). Congress needs to step up to its Constitutional responsibilities, and is complicit in any un-Constitutional acts by the President until it does.
In this case, President Trump (and I supported his candidacy after my first choice dropped out) has not articulated the same attack on Americans or our allies, nor an imminent or developing threat to America. Therefore, the cruise missile attack is a belligerent act, by any definition (we weren't responding to an attack on us or our allies, or to an imminent/developing threat). Therefore, President Trump needs a Congressional Declaration of War (or as a bare minimum, articulation of the national-interest reasons for doing so under the War Powers Act or an existing Authorization for the Use of Military Force).
Notwithstanding others' unfounded assertions that the American President has any sort of duty to citizens of any other nation (expressed nowhere at all in the Constitution), he indeed has no responsibility, duty, or other obligation of that kind. We The People simply DID NOT delegate that specific power to the Executive Branch. We could not if we wanted to - we have no such duty to other nations inherent in ourselves that we could delegate to our government.
Retired Army myself (over 32 yrs Active & Reserve, 3 mobilizations, 1 overseas deployment), with a son and a nephew on Active Duty, and a student of the Constitution. I highly recommend actually reading it, and the Federalist Papers, as a minimum. Hillsdale College has an excellent Constitution 101 course in 10 segments of an hour each to jump-start the process.
Either we are a Nation of Laws, or we are not.