President Trump has attacked Syria without congressional authorization, violating the US Constitution and the War Powers Resolution.
The War Powers Resolution is a series of barriers that Congress erected in the wake of the Vietnam War to defend the constitutionally-mandated role of Congress in deciding when the US will use military force if the US has not been attacked.
The "outer wall" of the War Powers Resolution is Section 2(c), which affirms that if the US has not been attacked, the president must receive congressional authorization in order to use force:
(c) The constitutional powers of the President as Commander-in-Chief to introduce United States Armed Forces into hostilities, or into situations where imminent involvement in hostilities is clearly indicated by the circumstances, are exercised only pursuant to (1) a declaration of war, (2) specific statutory authorization, or (3) a national emergency created by attack upon the United States, its territories or possessions, or its armed forces.
That's the "outer wall" that President Trump breached last night. In so doing, President Trump violated the Constitution and the War Powers Resolution.
However, fresh from the Vietnam experience, Congress was perfectly well aware that it was likely that a future US president would breach the "outer wall" of Section 2(c). So they erected other barriers within the War Powers Resolution. Like the "sixty days" wall of Section 5(b):
(b) Within sixty calendar days after a report is submitted or is required to be submitted pursuant to section 4(a)(1), whichever is earlier, the President shall terminate any use of United States Armed Forces with respect to which such report was submitted (or required to be submitted), unless the Congress (1) has declared war or has enacted a specific authorization for such use of United States Armed Forces, (2) has extended by law such sixty-day period, or (3) is physically unable to meet as a result of an armed attack upon the United States.
In the War Powers Resolution, Congress put a crucial tool in the hands of future members of Congress to help defend those two walls: the ability of a single member of Congress to introduce a "privileged resolution" -- one that can't be buried in committee, but can be forced by its sponsor to the floor for a vote -- to withdraw US forces from a conflict that hadn't been authorized by Congress.
SEC. 6. (a) Any joint resolution or bill introduced pursuant to section 5(b) at least thirty calendar days before the expiration of the sixty-day period specified in such section shall be referred to the Committee on Foreign Affairs of the House of Representatives or the Committee on Foreign Relations of the Senate, as the case may be, and such committee shall report one such joint resolution or bill, together with its recommendations, not later than twenty-four calendar days before the expiration of the sixty-day period specified in such section, unless such House shall otherwise determine by the yeas and nays.
(b) Any joint resolution or bill so reported shall become the pending business of the House in question (in the case of the Senate the time for debate shall be equally divided between the proponents and the opponents), and shall be voted on within three calendar days thereafter, unless such House shall otherwise determine by yeas and nays.
You can urge your representatives to invoke the War Powers Resolution to withdraw US forces from unauthorized conflict in Syria by signing the petition at MoveOn.