President Trump’s strike on Syria’s chemical weapons capabilities has ignited a new debate in Washington over the scope of the war on terror and just how far the administration can commit U.S. forces to an undeclared war in the Middle East.

Defense Secretary James N. Mattis said Friday that Mr. Trump authorized the strikes “as our commander in chief.”

“Under the Article II of the Constitution we believe the president has every reason to defend vital American interests and that is what he did tonight,” Mr. Mattis said.

SEE ALSO: U.S., allies launch missile attack on Syria in retaliation for using chemical weapons on civilians

Democrats, though, accused Mr. Trump of “unconstitutional” military action Friday, saying he should have come to Congress before he ordered strikes at Syria’s chemical weapons capability.

“Tonight’s U.S. military strikes on Syrian government targets are neither constitutional nor wise,” said Sen. Edward Markey, Massachusetts Democrat. “Attacks such as this on another country without congressional authorization are unconstitutional, and they push the United States closer to what could be an interminable, all-out conflict in Syria.

Sen. Jack Reed, ranking Democrat on the Senate Armed Services Committee, said Mr. Trump’s “impulsive tweets boxed him in” to having to take military action when long-term diplomacy would have been better.

“Put simply, the American people need to hear an actual strategy and an actual legal justification,” he said.

The Constitution makes the president the commander in chief, but grants Congress the power to declare war. There has always been tension between those two roles, but Mr. Trump’s claim that his powers cover military action for something as vague as “vital national interests” is likely to be heatedly debated.

Mr. Trump’s claim of “vital national interests” is similar to the claim of powers he cited in conducting a more limited strike last year, also against Syrian regime chemical weapons capabilities.

Mr. Mattis’s defense Friday night didn’t jibe with what CIA Director Mike Pompeo had told senators a day earlier. Mr. Pompeo, who is also a Harvard-trained lawyer, had said Mr. Trump had power to conduct the strike under the 2001 Authorization for the Use of Military Force that gave President George W. Bush the power to strike al Qaeda and other international terrorist organizations. That AUMF has been stretched to cover U.S. action in Libya, Yemen, Somalia, the Philippines and elsewhere.

Mr. Pompeo said that AUMF covered airstrikes like this, but said it would not cover an expansive combat commitment in Syria.

President Barack Obama also cited the 2001 AUMF when he first committed U.S. troops to assisting Syrian rebels in their fight against the Assad government.

The Senate is slated to begin a debate in committee on an updated AUMF later this month, and Mr. Trump’s action — and the chance of escalation — is likely to be a major part of that conversation.

Some Republicans backed the president’s claim of authority Friday.

“The administration is justified to take limited action in coordination with our allies to hold Assad accountable for the use of chemical weapons,” said House Foreign Affairs Committee Chairman Ed Royce.

While the administration didn’t request permission from Congress, it did notify key members.

Rep. Adam Schiff, the ranking Democrat on the House intelligence committee, said he was briefed by the CIA director about the evidence demonstrating the Syrian regime was behind last week’s chemical weapons attack that Mr. Trump was responding to.

Vice President Mike Pence also make phone calls to congressional leaders just ahead of Mr. Trump’s televised announcement Friday night.