While much of the world is in step with the actions of the Trump administration, some believe he skirted global law which promoted the hashtag. A spirited debate is expected.
"We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents", the USA president said in a televised address.
"Nothing is certain in these kinds of matters", he said.
The strikes were a culmination of a weeklong global push to punish the regime of President Bashar al-Assad after images and videos emerged Saturday from the Syrian city of Douma suggesting civilians-including children-had suffered in the attack. And finally, it's unclear if the United States will bomb Syria again if Assad's forces use chemical weapons once more.
Although the Western countries have all said for seven years that Assad must leave power, they held back in the past from striking his government with no wider strategy to defeat him.
Allied missiles struck at the heart of the Syrian chemical weapons arsenal, aiming to punish the Assad government for a suspected poison gas attack against civilians and deter the possible future use of such banned weapons.
Barrel bombs are large containers that are packed with fuel, explosives and scraps of metal. "But our efforts have been repeatedly thwarted" by Syria and Russian Federation, she said.
The military campaign was in response to a devastating chemical attack in the rebel-controlled Damascus suburb of Douma, which killed dozens of people earlier this month.
It concludes that there is "no plausible scenario other than that of an attack by Syrian armed forces".
Syria state TV, however, claimed that three civilians were wounded in Homs after "several" missiles were intercepted by Syrian air-defense systems.
USA officials said they gave Russian Federation no specific warnings of the attacks or the targets, but used the usual hotline with Moscow's military to ensure the airspace was clear.
After listening to scathing remarks delivered by Russian Ambassador to the UN Vassily Nebenzia about America's "hooliganism" and "illegal" actions against the Syrian regime, the USA responded by saying that the "time for talk ended last night". Last year's strike involved 59 missiles.
Britain launched cruise missiles as part of the co-ordinated military operation with the United States and France in response to the chemical weapons attack in Douma one week ago. The U.N. Security Council was meeting at Moscow's request.
Putin added that the strike had a "destructive influence on the entire system of global relations". She urged Russian Federation and Iran to help prevent any further use of chemical weapons by Assad's regime. Russian Federation said its experts found no trace of chemical agents.
While Trump, along with French and British allies, insisted that airstrikes were necessary to deter Syria's use of chemical weapons, Russian Federation maintains that there's no evidence chemical agents were used.
Syrians crowded onto the streets in noisy demonstrations of defiance afterward and their ally Russian Federation denounced the attack.
Theresa May is facing a furious backlash from MPs after she ordered United Kingdom forces to join the U.S. and France in targeted airstrikes on Syrian chemical weapons facilities early - without having gained the consent of parliament.
Allegations of Assad's chlorine use are frequent in Syria's conflict, raising questions about whether Washington had lowered the threshold for military action in Syria by deciding to strike after a chlorine attack.