President Obama on Wednesday ordered a sharp escalation of the U.S. military campaign against the Islamic State, authorizing U.S. airstrikes in Syria along with expanded airstrikes in Iraq as he vowed to wipe out the group’s terrorists “wherever they exist.”

“Our objective is clear: we will degrade, and ultimately destroy, ISIL through a comprehensive and sustained counter-terrorism strategy,” Obama said during a prime-time address to the nation.

The president’s address, just 15 minutes long, effectively completed a dramatic turnaround from the administration’s approach to the Islamic State just a few months ago, when the president downplayed the group’s advances through northern Iraq. Now, he is outlining a “comprehensive” strategy for targeting the organization in Iraq and Syria, including by potentially aiding moderate factions of the Syrian opposition.

Republicans who have urged a tougher approach voiced some relief that Obama appeared to be backing off “earlier dismissals” of the group’s fighting prowess. “He has finally begun to make the case the nation has needed him to make for quite some time: that destroying this terrorist threat requires decisive action and must be the highest priority for the United States and other nations of the free world,” House Speaker John Boehner said in a statement.

Boehner, though, cautioned that a “speech” is not a “strategy” and raised concerns that Obama is still treating this as an “isolated counterterrorism campaign.”

The president nevertheless made clear he is expanding the military campaign, and said the U.S. will lead a "broad coalition" against ISIS.

Until now, U.S. airstrikes have been limited to specific missions in northern Iraq. But Obama said Wednesday he is expanding those efforts “beyond protecting our own people and humanitarian missions, so that we're hitting ISIL targets as Iraqi forces go on offense.”

In a nod to his plans to go into Syria, Obama said: “I have made it clear that 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.”

Senior administration officials, on a conference call before the speech, said the president has authorized those airstrikes, and the military “will take action on the Syrian side” of the border.

The administration is not giving a timeframe for when the expanded airstrikes will begin. But senior administration officials said the militants operate freely across both sides of the Iraq-Syria border, and the U.S. cannot be restrained by that border in going after the organization.

Obama also announced Wednesday that another 475 service members will deploy to Iraq.

The president, who campaigned on ending the Iraq and Afghanistan wars, stressed Wednesday that this military effort will be different from those protracted conflicts and will not involve U.S. combat troops on the ground.

But, after weeks of mixed signals from the administration over how deeply it would commit to combating the Islamic State, or ISIS, the president set a goal Wednesday of eradicating the group.

“This counter-terrorism campaign will be waged through a steady, relentless effort to take out ISIL wherever they exist using our air power and our support for partner forces on the ground,” Obama said. In making the case, he cited the threat of Americans and other foreign fighters in the region returning to their home countries to “carry out deadly attacks.”

The president’s approach also involves a renewed call to train and arm moderate Syrian rebels. The president asked Congress to authorize this sort of program earlier this year, but the plan has not garnered support on Capitol Hill. He called in his address again for Congress to authorize and fund a mission to “train and equip these fighters.”

The call touched on the geopolitical tightrope the president is walking as he expands the anti-ISIS push into Syria. By going after the Islamic State on that side of the border, he is inadvertently aiding Bashar Assad, whose ouster Obama once sought. But Obama stressed Wednesday night that “we cannot rely on an Assad regime that terrorizes its own people” and must instead “strengthen the opposition as the best counterweight to extremists like ISIL, while pursuing the political solution necessary to solve Syria's crisis once and for all.”

House Republicans, in light of the president’s speech, have delayed a vote on a spending bill in order to consider the latest request.

The White House on Wednesday also announced another $25 million in military aid to the Iraqi government, while the Treasury Department outlined efforts to go after the Islamic State’s finances.

The speech marks the president’s highest-profile effort yet to both explain his strategy to the American people and rally support – in Congress and on the world stage – for an expanded campaign against the Islamic State.

Obama for weeks has struggled to articulate his policy toward ISIS and get on the same page with his own military advisers about how urgent the threat really is outside of Iraq and Syria.

Defense Secretary Chuck Hagel last month said the terror group is “beyond anything that we’ve seen.”

Obama, though, then confounded lawmakers when he openly admitted his administration didn’t have a strategy yet for confronting the threat in Syria. He sent more mixed messages when he said the goal is to “destroy” the group but also make it a “manageable problem.”

Since then, the president has projected a more consistent message that the Islamic State must be destroyed.

But while lawmakers on both sides have been clamoring for a clear strategy from the president, it remains to be seen whether they will go along with his plan. Some have demanded a vote on authorization for more airstrikes – which Obama’s advisers have suggested they don’t need – while others have shown reluctance to approve more aid for the Syrian opposition.

Sen. John McCain, R-Ariz., who supports aiding the opposition, told Fox News that task will be “extremely difficult” given how long the administration has waited.