We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is [if] we start seeing a whole bunch of chemical weapons moving around or being utilized. That would change my calculus. That would change my equation.

President Obama on Aug. 20, 2012

Twelve months later, the death toll in the Syrian civil war had risen to 100,000, and another two million had fled the country.

In March, the Assad regime and the rebel forces had accused each other of launching a chemical attack in Aleppo in which more than two dozen people died. But not until June 14 did the White House acknowledge that Assad had used chemical weapons. The administration promised the rebels “military support,” which it never sent.

Then, on Aug. 21, videos of a large-scale chemical attack began streaming out of Ghouta, a village in eastern Damascus. The footage showed rows of dead children and patients convulsing in their beds. An estimated 1,500 people died.

The red line had been crossed.

Within days, the news media were deluged with leaks that Obama planned an imminent attack on Syria.

On Monday, Aug. 26, Secretary of State John Kerry gave a rare speech at the State Department in which he said the use of chemical weapons by the Syrian government was “undeniable” and a “moral obscenity.”

Then, on Friday, Aug. 30, Kerry made an impassioned defense of unilateral military action, calling the chemical attack a “crime against humanity.”

But that night, Obama changed his mind. Lacking the support of the United Nations Security Council, Britain and the Arab League, he told White House aides that he wanted the backing of the Congress. After a two-hour meeting, he called Kerry and other cabinet members and broke the news.

In one stroke, Obama converted a limited strike on Syria into a test of America’s credibility in the world.

He also turned a dicey political move into a win-win for himself. If the Congress supported the attack, then the lawmakers shared responsibility in case it touched off a military backlash or a diplomatic crisis in the Middle East. If the Congress voted against it, then Obama could claim he was seeking the humanitarian high ground but was thwarted by the lawmakers.

“He decided, ‘Look, I have to make this America's red line, I have to make it Congress's red line,’” said Douglas Brinkley, a historian at Rice University who is sympathetic to the president.

Critics who have long pushed for intervention in Syria took a more cynical view the president’s motives. As the Wall Street Journal gibed in an editorial, “The reason to do this and authorize the use of force is not to save this President from embarrassment. It is to rescue American credibility and strategic interests from this most feckless of Presidents.”

Obama’s change of heart set up an internecine battle in the GOP, between hawks and anti-interventionists. He also created a hard dilemma for liberal Democrats, who would be disinclined to support military action but under heavy pressure to support their president (see p. 10).

Politicians on both side of the aisle agreed on one thing: Obama’s decision to seek congressional approval was sharply out of character.

When it suited his purposes, the president never hesitated to bypass Congress and act unilaterally on everything from health care to immigration reform to military action in Libya.

As a reluctant warrior who rose to power on promises of disentangling the U.S. from military engagements in the Middle East, he found himself trapped by his own red-line ultimatum and forced to order a military response he had spent more than two years trying to avoid.

Since the Arab Spring and the uprising in Syria that began in the spring of 2011, Assad repeatedly flouted Obama's calls to end the “outrageous” violence against the Syrian rebels and institute democratic reforms. Meanwhile, Russia and China stymied any type of United Nations response.

After the Aleppo attack in March, the president put off military action time and time again by saying he needed more evidence that chemicals were deployed. But the macabre video Images from Ghouta were impossible for the U.S. and its allies to ignore.

At first, British Prime Minister David Cameron seemed to be leading the charge for action. But on Thursday, Aug. 29, Parliament voted down a resolution to support military action.

"It is very clear tonight that, while the House has not passed a motion, it is clear to me that the British parliament, reflecting the views of the British people, does not want to see British military action,” Cameron said..

"I get that, and the government will act accordingly."

In the face of similar opposition from the American public and deep skepticism on Capitol Hill, Obama took his now-famous walk with Chief of Staff Denis McDonough in which he decided to follow Britain's lead and seek congressional approval before deciding whether to act alone.

Opposition leaders in Syria were disappointed and disillusioned while hawkish Republicans trashed the move as more evidence of a president incapable of executing a military strike alone or making a decision in a timely way.

“This is nothing short of an unmitigated disaster,” said Rep. Devin Nunes, a California Republican who has served on the House Intelligence Committee for the past three years.

Sen. Rand Paul, the Tea Party Republican from Kentucky, re-phrased Kerry's words about Vietnam. Kerry famously asked, “How do you ask a man to be the last to die for a mistake?” Paul turned this on its head: “How do you ask a man to be the first to die for a mistake?”

Democratic strategists countered by arguing that Obama was pulling the nation together on Syria.

“So, this is a president who is acting very deliberately and I think believes our coalition and the breadth of it is strengthened when not just the commander-in-chief but the representatives of the people weigh in on behalf of the intervention of the U.S. military,” former Obama press secretary Robert Gibbs said on NBC's "Meet the Press."

Obama's punt to Congress was a radical departure from how presidents have interpreted the War Powers Act for decades (see p. 11), and promised to have ripple effects for years to come. It also earned rebukes from a number of Middle East experts across the political spectrum.

“I am disappointed on the way it's been handled,” said Fuad Suleiman, a veteran U.S. diplomat who now teaches at St. Mary's College in Maryland. “I would have rather he had waited for the report by the U.N. observers before making any type of decisions ... and had developed a more comprehensive policy towards the Syria situation to begin with that had not put himself in such a box.”

Suleiman, a registered Republican, said Obama drew an artificial red line when he did not have to, eliminating the possibility of negotiating with regional powers and Russia on a successor to Assad who would not use chemical weapons.

“One of the essentials of a good foreign policy is not to box yourself in but to try to box in your opponent,” he said.

Bernard Finel, an associate professor at the National War College who specializes in use of force and counterterrorism strategies, said the U.S. has yet to get a clear picture of the players in the rebel opposition army, their ties to al Qaeda and their intentions if they ever successfully overthrow Assad.

“[The Obama administration] is wrapped around the axle about the red line, and they are trying to preserve their credibility, but it's hard to see what a strike accomplishes other than get Assad and his allies, who were already in the driving seat, even more intent on beating down the rebels and winning.”

Some lawmakers, still stung by the failure to turn up weapons of mass destruction in Iraq, questioned U.S. evidence linking the chemical attack outside of Damascus directly to Assad after several classified and unclassified briefings.

But Sen. John McCain, R-Ariz., who has spent months haranguing Obama to take military action to topple Assad, signaled support for the president's less-muscular strikes after spending the afternoon with Obama at the White House. McCain said it would “catastrophic” if the Congress did not back the president.

Obama got early heat from the Left, as some of his staunchest allies rejected the first administration resolution as too open-ended and asked the White House to more clearly limit its scope and aims.

The qualms on Capitol Hill suggested that even Obama's slow-walking on Syria may be too quick of a response to another Middle East conflict for many war-weary lawmakers.

The big showdown could come in the Senate, where winning a filibuster-proof majority could be a very close call.

Senior Congressional Correspondent David Drucker contributed to this report.