The coordinated strike marked the second time in a year that Trump has used force against Assad, who United States officials believe has continued to test the West's willingness to accept gruesome chemical attacks. A joint military operation, possibly with France rather than the U.S.in the lead, could send a message of worldwide unity about enforcing the prohibitions on chemical weapons.
Trump called the incident a "heinous attack on innocent" Syrians and vowed that the United States would respond: "This is about humanity; it can't be allowed to happen".
French President Emmanuel Macron says his nation, the United States and Britain have launched a military operation against the Syrian government's "clandestine chemical arsenal".
The samples were said to have suggested the presence of both chlorine gas and an unnamed nerve agent, with the officials saying they were "confident" in the intelligence, although not 100% sure.
Trump indicated the strikes would continue until the Syrian regime's use of chemical weapons ends.
"The goal of our actions tonight is to establish a strong deterrent against the production, spread and use of chemical weapons", Trump said.
The announcement came on Thursday, just days after the suspected attack which, according to activists and medics on the ground, claimed the lives of more than 85 civilians and injured at least 1,200 people.
Why is the West considering military action?
Inspectors with the Organization for the Prohibition of Chemical Weapons (OPCW) as of Thursday were on their way to Syria, and are scheduled to begin their work Saturday, further complicating the timing for any strike.
Asked about the risks of USA military retaliation, Mattis cited two concerns, starting with avoiding civilian casualties.
The presence of Russian troops and air defenses in Syria were among numerous complications weighing on Trump, who must also consider the dangers to roughly 2,000 American troops in the country if Russia were to retaliate for US strikes.
"On a strategic level, it's how do we keep this from escalating out of control", he said.
A US official told Reuters the strikes were aimed at multiple targets and involved Tomahawk cruise missiles.
"No amount of American blood or treasure can produce lasting peace and security in the Middle East".
Trump calls the two countries those "most responsible for supporting, equipping and financing the criminal Assad regime". Russia's ambassador to Lebanon said any missiles fired at Syria would be shot down and the launching sites targeted - a stark warning of a potential major confrontation.
"I'm not ready to speculate that that would happen", Mattis said.
At the House hearing, Rep. Tulsi Gabbard, D-Hawaii, disputed Trump's legal authority to act without congressional authority and suggested a USA strike would lead to war with Russian Federation.