President Barack Obama, in Tuesday night’s televised speech from the White House, called for a postponement of Congress’ vote to authorize U.S. military action against Syria.

But he also persisted in making an unconvincing case that “with modest effort and risk” American air power could “deter” and “degrade” the Syrian government’s use of chemical weapons.

Mr. Obama is correct in decrying the Assad regime’s use of sarin gas against civilians as “not only a violation of international law” but “a danger to our security.” He fairly warned that if Syria isn’t called to account for its Aug. 21 poison-gas massacre of more than 1,400 civilians, “other tyrants will have no reason to think twice about acquiring poison gas and using them.”

However, the president failed again Tuesday night to provide sufficient answers to fundamental questions about how the limited military response he favors would hold Syria to effective account.

And earlier Tuesday, Syria’s stated acceptance of a Russian proposal that it turn over its chemical weapons further clouded the issue. Or was that development a ray of sunshine-like hope?

Russian officials suggested that move Monday after U.S. Secretary of State John Kerry, in answer to a reporter’s question in London, said Syrian President Bashar al-Assad could still avert a U.S. attack.

How? According to Mr. Kerry, by turning over “every bit of his weapons to the international community within the next week, without delay.” The secretary added: “But he isn’t about to.”

Then on Tuesday night, President Obama cited Syria’s diplomatic overture as grounds to delay Congress’ vote authorizing armed force. But he pointed out: “It’s too early to tell whether this offer will succeed.”

Unfortunately, though, it’s too late to erase the many dangerously mixed messages the Obama administration has sent about Syria.

President Obama persisted in sowing confusion Tuesday night. He repeated his pledge that the U.S. air strike would be “targeted” and “limited.”

That, of course, suggests that its impact would be limited, too.

Indeed, Secretary Kerry said Monday that an American attack in would be an “unbelievably small, limited kind of effort.”

On Tuesday night, President Obama tried again to counter that comment by telling the nation: “The U.S. military doesn’t do pinprick strikes.”

But we also won’t do “boots on the ground” in Syria, as the president vowed again Tuesday night. That mantra doesn’t just reassure the American people. It reassures Mr. Assad.

Still, there’s nothing reassuring about the administration’s conflicting statements on Syria.

On Aug. 20, 2012, President Obama warned that the Syrian government must not cross a “red line” by using chemical weapons.

Last week Mr. Obama hedged on that, saying, “It’s not my red line, it’s the world’s red line.”

On Aug. 18, 2011, President Obama said, “For the sake of the Syrian people, the time has come for President Assad to step aside.”

On Tuesday night, he said: “I don’t think we should remove another dictator with force.”

In other words, our commander in chief is prepared to unleash U.S. air power on Mr. Assad, but doesn’t want to oust him from power.

If all this sounds muddled, that’s because it is.

The president also has not ruled out ordering an air assault on Syria even if federal lawmakers vote don’t give him a green light to do so. But barring a better case than the president presented on Tuesday night, Congress looks unlikely to approve that resolution, even with a postponement in the vote. Moreover, polls show that the American public is strongly opposed to attacking Syria.

And with or without U.S. military intervention, if regime change does occur in Syria, what then? During the 2½-year civil war there, the rebel forces have been increasingly infused with al-Qaida-affiliated terrorists, many of them from outside Syria.

That’s a major concern on this 12th anniversary of the 9/11 attacks on America — or on any other day.

An even more pressing concern: President Obama’s seemingly undaunted intent to attack Syria absent a supposedly verifiable deal with the Assad regime and Russia.

The president has not made a persuasive argument that such a military mission would produce the desired result. If Congress eventually votes on his proposed use of force, it should reject it.

And President Obama should not defy the will of the legislative branch — and of the American people — by attacking Syria.