President Barack Obama, having drawn a “red line” in the sand and then seeing Syria cross it, is making preparations for a military response involving air strikes. In doing so, he is discussing with America’s allies how to proceed.

He should first be consulting with those who represent the American people in Congress.

The president should not take unilateral action that could commit the nation to another military adventure in the Middle East.

There is no question that the Assad regime had already passed the bounds of international law before the Aug. 21 poison gas attack that left at least 100 civilians dead.

The death toll in the civil war has passed 100,000, many of them civilians who have died at the hands of Syria’s military.

President Obama, however, has declared the gas attack as the specific reason for a U.S. military response. But Congress has yet to sign on to his plan.

Though the president does have the legal authority to take military action unilaterally in an emergency, the situation in Syria does not meet that standard.

A growing number of House members are urging that Mr. Obama consult with Congress on any pending action against Syria, as required by the Constitution and the War Powers Resolution of 1973.

In a letter to the president, they cite the administration’s willingness to engage in military action in what it presumes to be “the national interest.” But absent congressional approval, such action is a violation of the constitutional separation of powers, they say.

This is not a partisan effort by a Republican-run House. It’s a bipartisan appeal to the president to respect legal congressional prerogatives.

The lawmakers cite Mr. Obama’s rationale to engage in NATO’s 2011 air strikes against Libya to help depose Moammar Gadhafi.

The White House then insisted that congressional authorization was not required because the military was not engaged in “hostilities.”

The House members wrote: “If the use of 221 Tomahawk cruise missiles, 704 Joint Direct Attack Munitions and 42 Predator Hellfire missiles expended in Libya does not constitute ‘hostilities,’ what does?”

Good question.

First District Rep. Mark Sanford, R-S.C., who is among those who have signed the letter, said the nation deserves to have Congress weigh in as the president prepares to engage yet another nation in the Mideast.

“I don’t think that anybody is less than appalled with what’s going on,” Rep. Sanford told us Thursday, referring to the poison gas attack. “The question is, how do you make it right? There shouldn’t be unilateral action by the executive branch in getting us involved.”

Congress hasn’t declared war since World War II, but has acted to give the executive branch the go-ahead on military action since, most recently in Afghanistan and Iraq.

The arguments for congressional authorization are certainly well-known to the president and his administration.

As a White House candidate in late 2007, then-Sen. Obama told The Boston Globe that executive military action without congressional approval can’t be warranted “in a situation that does not involve stopping an actual or imminent threat to the nation.”

Vice President Joe Biden was just as emphatic on the issue as a senator in 2007, saying on MSNBC’s “Hardball” that only “Congress [has] the power to initiate hostilities.”

Both Mr. Obama and Mr. Biden were right — then. While our nation’s founders made the president the commander in chief, they checked that power by giving Congress the constitutional authority to approve — or disapprove — the use of American military might.

The law of the land still requires that the president, barring an emergency, obtain such approval.

President Obama now appears convinced that America should attack Syria.

But before giving that order, he should convince federal lawmakers of the need for an armed intervention by the United States.