"We're going to do what's necessary to take the fight to this terrorist group," Obama said in remarks from the White House lawn.
After his comments, he immediately left on Marine One to begin his trip to the United Nations General Assembly in New York, where he will work to secure more international support for the fight against ISIS. [Read: Letter from the president -- War Powers resolution regarding Syria]
A group of Arab countries — Bahrain, Jordan, Saudi Arabia, Qatar and the United Arab Emirates — participated in or supported the strikes, according to the administration, a key diplomatic coup for the White House.
"America would act as part of a broad coalition and that's exactly what we've done," the president said, adding that "this is not America's fight alone."
"America is proud to stand shoulder to shoulder with these nations on behalf of our common security," he added.
According to the Pentagon, the U.S. used a combination of manned aircraft, drones, and Tomahawk missiles to conduct 14 airstrikes against ISIS targets within Syria. The strikes destroyed or damaged ISIS training compounds, headquarters buildings, storage facilities, a finance center, and supply trucks and armed vehicles, and were targeted around the city of Raqqa.
Separately, the U.S. also targeted an al Qaeda group that had set up camp in Syria. According to the Pentagon, the strikes targeted the Khorasan Group, a gathering of al Qaeda veterans who had gathered west of Aleppo and were plotting an "imminent attack" against the United States and Western interests.
That group of eight airstrikes, executed exclusively by the U.S., destroyed training camps, and explosives and munitions production facility, a communication building, and command and control facilities.
The president said the action was to "disrupt plotting" among "seasoned al Qaeda operatives."
"Once again it must be clear to anyone who would plot against America and try to do America harm that we will not tolerate safe havens," he said.
Deputy national security adviser Ben Rhodes said the administration had been tracking the Khorasan Group's plotting of an attack on the U.S. homeland "for some time" and that the al Qaeda members had "clear ambitions to launch external operations against the United States."
"We believe that attack planning was imminent," Rhodes said.
The strikes have been broadly applauded on Capitol Hill, where there has largely been bipartisan support for the president's plans to expand the air campaign against ISIS.
"This is one step in what will be a long fight against ISIL. With strong coalition partners, a capable military, and a clear mission; it is a fight we can win," House Armed Services Chairman Buck McKeon (R-Calif.) said in a statement.
Obama said there was "bipartisan support for the action that we are taking."
"America is always stronger when we stand united," Obama said.
But the expanded military effort has also renewed calls from some members to have Congress explicitly authorize the strikes within Syria with a vote.
"I think the mission has now switched to an offensive mission," Sen. Tim KaineTimothy (Tim) Michael KaineDemocrats call on Biden administration to ease entry to US for at-risk Afghans Manchin, Sinema join GOP to sink filibuster change for voting bill Desperate Dems signal support for cutting Biden bill down in size MORE (D-Va.) told MSNBC. "Clearly I think Congress should be weighing in."
President Obama gave the final orders authorizing the strike on Thursday, after meeting with military commanders to discuss the effort at U.S. Central Command in Tampa. According to Rhodes, the timing of the strikes was dictated by the concerns of military planners and the readiness of coalition forces.
"We didn’t time this related to the U.N. meetings," Rhodes said.
"We did not provide advance notification to the Syrians at a military level, or give any indication of our timing on specific targets," Psaki said, adding that the U.S. did warn Syria not to engage U.S. aircraft.
A senior administration official said last week that the U.S. would shoot back if Assad’s military shot at planes that entered Syrian airspace, and that the president could authorize the destruction of Syria’s air defense system.
On Tuesday, Obama said the U.S. also "will move forward with our plan...to ramp up our effort to train and equip the Syrian opposition."
Congress gave Obama authorization for that program as part of a short-term budget measure that passed last week, although National Security Adviser Susan Rice said the program would take “many months” to get running.
Congressional aides confirmed the president phoned Speaker John BoehnerJohn Andrew BoehnerDemocrats eager to fill power vacuum after Pelosi exit Stopping the next insurrection Biden, lawmakers mourn Harry Reid MORE (R-Ohio) and Minority Leader Nancy Pelosi (D-Calif.) on Monday to inform them of the strikes. Other administration officials, including Defense Secretary Chuck HagelCharles (Chuck) Timothy HagelInterpreter who helped rescue Biden in 2008 escapes Afghanistan Overnight Defense & National Security — Pentagon chiefs to Congress: Don't default Pentagon chiefs say debt default could risk national security MORE and Vice President Biden briefed the chairmen and ranking members of the defense and intelligence committees.
On Tuesday, Obama submitted a letter under the War Powers act officially notifying BoehnerJohn Andrew BoehnerDemocrats eager to fill power vacuum after Pelosi exit Stopping the next insurrection Biden, lawmakers mourn Harry Reid MORE and Senate Majority Leader Harry ReidHarry Mason ReidSchumer vows to vote on Biden Supreme Court pick with 'all deliberate speed' Democrats say change to filibuster just a matter of time The Hill's 12:30 Report - Presented by Connected Commerce Council - Biden faces reporters as his agenda teeters MORE (D-Nev.) he had ordered the strikes.
'[ISIS] is a direct threat to the safety and security of the United States and our allies,” Boehner said in a statement. “I support the airstrikes launched by the president, understanding that this is just one step in what must be a larger effort to destroy and defeat this terrorist organization. I wish our men and women in uniform Godspeed as they carry out this fight.”
Reid, in his statement, hailed the president's efforts at coalition-building.
"The presence of Arab nations in these airstrikes and President Obama's commitment that we will not use U.S. ground forces in combat are clear evidence that President Obama will not repeat the mistakes of the past," Reid said. “As we move forward, I expect consultations between the administration and Congress to continue.”
The president is expected to meet later Tuesday with a gathering of coalition nations hosted by Secretary of State John KerryJohn KerryOvernight Energy & Environment — Limits to contamination claims at military bases The Hill's Morning Report - Biden: Russia attack 'would change the world' Overnight Energy & Environment — High court will hear case on water rule MORE on the margins of the United Nations General Assembly, press secretary Josh Earnest said.
--This report was updated at 12:10 p.m.