"America's credibility is on the line here," Secretary of State John Kerry told Fox News on Sunday, making the case for a military strike against Syria. It would be more accurate to say Barack Obama's credibility is on the line. Given the president's reversals on crucial questions about foreign intervention, he does not have much to spare.

In a speech at a 2002 anti-war rally, Obama, then an Illinois state senator, conceded that Iraqi dictator Saddam Hussein was "a brutal man," "a ruthless man," "a man who butchers his own people to secure his own power." He noted that the Iraqi dictator "has repeatedly defied U.N. resolutions, thwarted U.N. inspection teams, developed chemical and biological weapons, and coveted nuclear capacity."

In short, there was no question that "the world, and the Iraqi people, would be better off without him." Still, Obama said, "Saddam poses no imminent and direct threat to the United States." Hence a U.S. invasion aimed at overthrowing him would be "a dumb war," "a rash war," "a war based not on reason but on passion."

Notably, Saddam's crimes against his own people included using chemical weapons against Kurds in northern Iraq, a campaign that killed some 5,000 men, women, and children. That murderous assault, in Obama's view, did not justify U.S. intervention.

Today, by contrast, Obama says a sarin-gas attack that caused about 1,400 of the 100,000 deaths so far in Syria's civil war demands an American response in the form of missiles aimed at President Bashar al-Assad's forces. "What message will we send if a dictator can gas hundreds of children to death in plain sight and pay no price?" Obama asked in a speech on Saturday. Presumably the same message he was willing to send when he opposed war with Iraq.

Don't worry, Obama said: "This would not be an open-ended intervention." That assurance is hard to believe, given that Obama's 2011 air campaign in Libya, which he likewise claimed was necessary to prevent the "slaughter" of civilians, quickly became an operation aimed at toppling Muammar al-Qaddafi.

Furthermore, Obama already has said Assad must go, and U.S. involvement in Syria already has escalated. Two months ago, Obama announced that he would start arming the rebels because Assad had crossed a "red line" by using chemical weapons. Now that Assad has used them again, Obama says, the U.S. has no choice but to launch a missile attack. And if Assad remains undeterred after that punishment, what then?

Unlike in the case of Libya, Obama has decided to seek approval from Congress before going to war with Syria. But he and Kerry have made it clear that they think consulting with the legislature, the only branch of government authorized by the Constitution to declare war, is optional.

"I have the authority to carry out this military action without specific congressional authorization," Obama asserted on Saturday. "The president of the United States has the right to take this action," Kerry told ABC News the next day. "[He] doesn't have to go to Congress."

Obama took a different position on the president's powers before he started wielding them. "The President does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation," he told the Boston Globe in December 2007.

That was Obama's written response to a questionnaire about executive power, so he had time to reflect on the implications of those words. Among them: If Congress declines to approve the use of military force against Syria when it votes on the question next week, Obama does not have the constitutional authority to proceed with an attack anyway.

Kerry, who appeared on several Sunday morning talk shows over the weekend, was repeatedly asked what his boss will do if Congress rejects military action against Syria. He dodged the question every time.