President Barack Obama, according to background briefings by his aides, reached a fateful decision late Friday afternoon as he strolled along the White House lawn with his chief of staff Denis McDonough. Contrary to every expectation by his national security team, Obama concluded that he should ask Congress for authorization to bomb Syria.

The full reasoning behind the president’s turnabout remains murky. He may have wanted to share responsibility for a risky strategy to punish the barbarous regime of Syrian strongman Bashar Assad for using chemical weapons against his own people. Obama may have recognized the political dangers of attacking another Middle Eastern country without popular support at home.

And the president, a former part-time constitutional law professor, may have also belatedly recalled the wording of Article One, Section Eight of the Constitution that grants Congress the sole power “to declare war.”

But whatever Obama’s underlying motivations and however the Syrian vote plays out on Capitol Hill, the president’s decision to go to Congress represents a historic turning point. It may well be the most important presidential act on the Constitution and war-making powers since Harry Truman decided to sidestep Congress and not seek its backing to launch the Korean war.

Just a few days ago, before Obama’s decision was known, legal scholars from both the right and the left were in agreement that waging war over Syria — no matter how briefly — without congressional approval would bend the Constitution beyond recognition.

Jack Goldsmith, a Harvard law professor who served as a Bush administration lawyer during the run-up to the 2003 Iraq war, wrote in the legal blog Lawfare, “The planned use of military force in Syria is a constitutional stretch that will push presidential war unilateralism beyond where it has gone before.” And liberal constitutional scholar Garrett Epps, writing for the Atlantic , concluded, “It’s pretty clear that an American attack would violate the Constitution.”

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Virtually no one in politics, the press or the academic community expected Obama to go to Congress for approval. That isn’t the way the presidential power works in the modern era. It is a sad truth that whoever occupies the Oval Office invariably expands rather than trims back the Imperial presidency. Obama himself has reflected this pattern with his aggressive enhancement of the National Security Agency’s efforts to monitor electronic communications.

For more than six decades, the war-making powers of Congress have been eviscerated by presidents of both parties.

Which brings us back to Truman, who in 1950 balked at asking a Congress weary after World War II for approval to militarily respond to the Communist attack on South Korea. Dean Acheson, Truman’s secretary of state, claimed in his memoirs that a congressional debate over the Korean War “would hardly be calculated to support the shaken morale of the troops or the unity that, for the moment, prevailed at home.”

Acheson may not have remembered that military morale and national unity are not mentioned in the Constitution. But the war-marking powers of Congress are at the heart of the nation’s founding document. It was as if the sign on Truman’s desk read, “The Buck Stops Here — And This Is Also Where the Constitution Is Twisted.”

The plain-spoken Truman resorted to weaselly words to claim that Korea was a United Nations-sponsored “police action” rather than a war. No other American “police action” has ever led to 54,246 wartime deaths.

Truman’s assertion of vast executive power as commander in chief set a template for future presidents. Even when presidents have gone to Congress for approval of major military engagements, these blank-check authorizations have often been based on deceptive arguments.

Lyndon Johnson premised the entire Vietnam War on the 1964 Gulf of Tonkin Resolution, which was designed to permit a limited response to two minor and maybe mythical naval skirmishes with North Vietnam. Similarly hyperbolic were George W. Bush’s claims about Saddam Hussein’s nonexistent arsenal of weapons of mass destruction.

Even more legally dubious were all the times a president sent troops and planes into combat without anything more than desultory briefings of the congressional leadership.

Ronald Reagan dispatched the Marines into Grenada in 1983 under the preposterous rationale that he was only protecting endangered American medical students. Bill Clinton skirted congressional approval for the 1999 airborne attacks to halt Serbia’s ethnic cleansing of Kosovo on the shaky grounds that this was a NATO operation. And Obama himself was even on flimsier footing when he justified America’s participation in the 2011 bombing campaign over Libya based on a United Nations resolution.

But Syria did not provide Obama with any of these fig-leaf justifications.

No American lives are in danger and the national security threat is hard to identify. Not only is NATO not participating, but also neither are the Brits, the United State’s closest diplomatic ally. With Russia serving as Assad’s enabler, there will be no Security Council resolution or U.N. mandate.

Every time a president employs questionable legal arguments to wage war, it becomes a valuable tool for the next commander in chief impatient with the constitutional requirement to work through Congress. That’s why it would have been so dangerous for Obama to go forward in Syria without a congressional vote or the support of the U.N. or NATO. It is as much of a slippery slope argument as the contention that Iran, say, would be emboldened with its nuclear program if America did not punish Assad’s chemical attacks.

Assuming Obama wins congressional approval, America’s coming attack on Syria is designed to set a lasting precedent: No government can ever again use chemical, biological — let alone nuclear — weapons without facing devastating consequences. As Obama asked rhetorically in his Saturday Rose Garden statement, “What message will we send if a dictator can gas hundreds of children to death in plain sight and pay no price?”

But Obama’s decision to seek congressional approval may prove to be an even more important precedent. Future presidents — as they consider unilateral military action without American security hanging in the balance — will have to answer, “Why didn’t you go to Congress like Obama did over Syria?”

Confronted with a series of wrenching choices over Syria, Obama chose the course that best reflects fidelity to the Constitution as written. Hopefully, in the days ahead, taking that less traveled road by presidents will make all the difference.