Most importantly, Trump Tweeted this:

What will we get for bombing Syria besides more debt and a possible long term conflict? Obama needs Congressional approval. — Donald J. Trump (@realDonaldTrump) August 29, 2013

Trump explicitly understood that a military response would require congressional approval. Yet Thursday, Trump ordered a strike on Syria without seeking that approval, citing a chemical weapons attack by the Assad regime. “Fifty-nine Tomahawk missiles were fired from American destroyers in the eastern Mediterranean at Al Shayrat airfield,” The New York Times reported.The cost in missiles alone was roughly $50 million.

There are those who supported the president’s actions.

Prior to the strike, various members of the military-industrial complex, hawkish pundits, and social-media users outraged by killings of Syrian civilians demanded that Trump do something in response to the abhorrent slaughter of innocents. But Trump never swore to slake a vocal minority’s outrage. He swore to uphold the Constitution. And if his 2013 statement on bombing Syria left any doubt as to whether he understood the proper role of Congress, he had lots of reminders prior to Thursday.

Back in 2013, “more than 100 House lawmakers––at least 98 Republicans and 18 Democrats––signed on to a letter formally requesting that President Obama seek congressional approval for any military response to the use of chemical weapons in Syria,” the Washington Post reported. “The letter, first written by Rep. Scott Rigell (R-Va.), suggests that failure to seek congressional authorization for military strikes would be unconstitutional.”

That warning was reasserted this week. Senator Mike Lee, a Utah Republican, put it this way:

If the United States is to increase our use of military force in Syria, we should follow the Constitution and seek the proper authorization from Congress. President Trump should make his case in front of the American people and allow their elected representatives to debate the benefits and risks of further Middle East intervention to our national security interests.

Senator Rand Paul, a Kentucky Republican, said “the president needs congressional authorization for military action as required by the Constitution, and I call on him to come to Congress for a proper debate." He added, "Our prior interventions in this region have done nothing to make us safer and Syria will be no different."

Senate Foreign Relations Committee Chairman Bob Corker told CNN’s Jake Tapper that President Trump should “certainly come to Congress” before acting in Syria.

And elected officials were not alone.

“If Article I, Section 8, Clause 11 of the United States Constitution means anything, it means that the president must obtain congressional approval before taking us to war against a sovereign nation that has not attacked the U.S. or its allies and is not threatening to attack the U.S. or its allies,” declared National Review’s David French, who served in the Judge Advocate General's Corps during the Iraq War.