He used a rare prime-time speech to lay out a program that would have been unthinkable until recently: a multi-country fight in the Middle East, involving American airstrikes and special forces in Iraq and now Syria, as part of an open-ended counter-terrorism plan, without an end date.
"We will degrade, and ultimately destroy, ISIL through a comprehensive and sustained counter-terrorism strategy," Obama said.
"We will hunt down terrorists who threaten our country wherever they are. That means I will not hesitate to take action against ISIL in Syria, as well as Iraq. This is a core principle of my presidency: If you threaten America, you will find no safe haven."
Obama carefully avoided calling it a war.
He added a familiar caveat, as he has at every step of the way in recent weeks as American military involvement in the region steadily increased, promising once again that combat troops won't be going back in: "We will not get dragged into another ground war in Iraq."
Canada is among the countries involved in the Iraq mission, but the federal government has quickly shot down the idea of participating in Syria. Late Wednesday, a spokesman for Prime Minister Stephen Harper said Canada was committed to addressing this "barbaric terrorist threat," with humanitarian aid and military advisors in Iraq, but said the commitment would be re-evaluated after 30 days and would not expand to Syria.
Obama's dual position — yes to airstrikes, no to a ground war — appears to be in line with American public opinion.
Attitudes have hardened lately, with an increased willingness to launch strikes against the Islamic State of Iraq and the Levant — first in Iraq, and now in Syria. But polls also suggest that they still staunchly oppose another ground war, after a decade of bloody and multi-trillion dollar foreign conflicts.
Wednesday's speech was the latest step down an improbable path for a presidency that sprang from an anti-war platform and received a Nobel Peace Prize in its infancy.
The catalyst appeared to be brutal images from the Islamic rebellion that has gobbled up territory in eastern Syria and western Iraq, notably two videos of American journalists being beheaded.
American public opinion has shifted, even from just a few weeks ago.
An NBC/Wall Street Journal poll said 61 per cent declared attacking ISIL in Iraq and Syria would be in the national interest, versus a minuscule 13 per cent who disagreed. A year ago, respondents answered with a deafening No when asked whether the U.S. should attack Syria's Assad regime — only 21 per cent supported action in Syria in August 2013.
After avoiding participation in the Syrian conflict, Obama had briefly flirted with airstrikes last year upon evidence that the regime had used chemical weapons. But he was shouted down by American public opinion back then.
Things began changing in June, as ISIL swept through Iraq. Three years after he withdrew American troops to much fanfare, Obama announced he was sending up to 300 military advisors back into the country. Then in August, he announced airstrikes to defend American interests and stranded refugees there. Last week, he deployed 350 more military personnel.
Finally, his series of announcements Wednesday included: 475 military personnel to help Iraqi and Kurdish forces with training, intelligence and equipment; open-ended airstrikes in Syria, as well as Iraq; and a UN Security Council meeting this month to deal with hundreds of fighters with Western passports.
There was a strong word of caution from a surprising quarter Wednesday: the Canadian speechwriter who penned George W. Bush's speeches a decade ago, as he prepared to invade Iraq. David Frum, the author of the "axis of evil" line, said he's haunted by the decision to launch a war under false pretenses and said there's not a single day that he doesn't think about it.
To illustrate his argument, he pointed to the 180-degree shift from 2013. Obama has not only switched his main target in Syria, from the regime to the rebels, but he's also gone from insisting on the need to get congressional approval last year to now insisting he can act without it.
"It seems like only last year that this president was asking Congress for authority to bomb Assad. Twelve months later, he will bomb Assad’s enemies," Frum wrote in the Atlantic.
"Why does bombing one side of a war require congressional permission, while bombing the other side does not? The administration doesn’t answer, because nobody is asking. Something must be done! This is something! Let’s do this!... It's a reaction: an emotional reaction, without purpose, without strategy, and without any plausible — or even articulated — definition of success."
But Frum may be in the minority.
The ISIL threat has already shaken up the early race for the 2016 presidential nominations. The most prominent war-skeptic in the Republican party, Sen. Rand Paul, has begun making more hawkish noises.
Canada's ambassador to the U.S. acknowledged the difficulty of Obama's position, with respect to Syria.
"Canada didn't arm moderates in Syria a couple of years ago because we didn't know who they were. And now we find that the people who were armed last year, all the arms are owned by ISIS," Gary Doer said earlier in the day, during an energy-industry event in Calgary.
"So these are not easy questions for the president."
Hours before the speech, Obama convened a meeting with more than a dozen members of his national-security team at the White House. He also spoke by phone with King Abdallah Abd al Aziz of Saudi Arabia.
The U.S. has pressured regional allies to step up their own role in fighting ISIL, and Obama has expressed exasperation at the notion that Americans should be expected to do all the heavy lifting.
He ended his speech by warning that the fight could be long.
But he added, again: this isn't a war.
"It will take time to eradicate a cancer like ISIL," Obama said.
"And any time we take military action, there are risks involved... But I want the American people to understand how this effort will be different from the wars in Iraq and Afghanistan. It will not involve American combat troops fighting on foreign soil. This counterterrorism campaign will be waged through a steady, relentless effort to take out ISIL wherever they exist, using our air power and our support for partners' forces on the ground."