It is not about regime change.
The strikes were carried out by manned aircraft, including France's and ships that launched cruise missiles from the Mediterranean Sea.
The strikes were aimed at "chemical weapons infrastructure" in what the United States billed as a warning against Mr Assad employing such weapons in the future - a warning he has not heeded in the past.
Mattis acknowledged that the United States waged the attacks only with conclusive evidence that chlorine gas was used in the April 7 attack in Syria.
The Damascus sky lights up missile fire as the US launches an attack on Syria targeting different parts of the capital early Saturday, April 14, 2018.
Defense Secretary James Mattis appeared to have referenced Russian Federation and warned of its "disinformation campaign" in the coming days.
The move comes after days of warnings by the USA president to the Assad regime and its supporter, the government of Russian President Vladimir Putin. The Pentagon said it would provide a more detailed assessment on Saturday morning.
The statement made no reference to whether Parliament would be given a say on military action - prompting renewed concerns among opposition parties and some Tory MPs that Mrs May is prepared to go ahead without a Commons vote.
In the days leading up to the US attack, Russian Federation had warned that it would defend its troops in Syria.
He says there is "no doubt" that the Syrian government is responsible. "To Iran and Russian Federation, I ask you: What kind of a nation wants to be associated with the mass murder of innocent men, women and children?" he said.
Congressional leaders are supporting the president's decision to launch airstrikes in retaliation for an apparent chemical attack against civilians - although there are some reservations.
Abdul-Hamid Alyousef, 29, holds his twin babies who were killed during the suspected chemical weapons attack in Khan Sheikhoun in the northern province of Idlib, Syria, on April 4, 2017. He said Syrian surface to air missile batteries had attempted to fire back, but there were no initial reports of any allied losses.
In a robust response to the alleged use of chemical weapons by Syria, the US, UK and France struck targets associated with those weapons in the area of Damascus yesterday.
British Prime Minister Theresa May said she had authorized British armed forces "to conduct coordinated and targeted strikes to degrade the Syrian regime's chemical weapons capability".
General Joseph Dunford, Washington's top general, said the precision strikes hit a scientific research center near Damascus, a storage facility and command post also near the capital and a chemical weapons storage facility near Homs.
In his nationwide address, Trump stressed that he has no interest in a longtime fight with Syria.
"In the last century, we looked straight into the darkest places of the human soul", Trump said. "And great warriors they are".
"We specifically identified these targets to mitigate the risk of Russian forces being involved", Dunford told reporters, adding the USA military advised Russia of airspace that would be used in the strike but did not "pre-notify them".
Douma and the sprawling eastern Ghouta region near the capital, Damascus, had been under rebel control since 2012 and was a thorn in the side of Assad's government, threatening his seat of power with missiles and potential advances for years.
Senate Armed Service Committee Chairman John McCain is applauding the airstrikes but said "they alone will not achieve US objectives in the Middle East". In a statement Friday night, May called last weekend's chemical attack "pure horror".