The Prime Minister insisted "every possible diplomatic channel" had been explored to avoid armed conflict, but that the decision by Russian Federation - which backs Assad - to veto an independent United Nations investigation into the Douma attack meant military action was unavoidable.
"A short time ago, I ordered the United States armed forces to launch precision strikes on targets associated with the chemical weapons capabilities of Syrian dictator Bashar al-Assad", Trump said, in a prime-time address.
Russian Federation have said that they would take any attacks very seriously and the world is waiting to see what will happen next.
Russian Federation on Saturday called for an emergency meeting of the United Nations Security Council as Moscow said it would consider supplying S-300 missile systems to Syria following USA -led strikes.
The allied operation comes a year after the U.S. missile strike that Trump said was meant to deter Assad from further use of chemical weapons.
Trump also spoke of attacks dating back to past year, when Trump says Assad launched an attack "against his own innocent people".
"We can not tolerate the recurring use of chemical weapons, which is an immediate danger for the Syrian people and our collective security", a statement from the Elysee presidential office said.
"But maybe not." Trump added that the U.S.
Residents in Damascus were caught off guard by the strikes, independent reporter Danny Makki told NPR, speaking from Damascus.
The Syrian Observatory for Human Rights monitoring group said that "scientific research centers" and "several military bases" were hit in the strikes.
"We were very precise and proportionate, but at the same time it was a heavy strike", he said.
"No amount of American blood or treasure can produce lasting peace and security in the Middle East", Trump said.
He called them "most responsible for supporting, equipping and financing the criminal Assad regimen".
Syria "condemns with the strongest terms the tripartite US, British and French aggression on Syria as being a flagrant violation of the worldwide law and the principals of United Nations charter", SANA news agency quoted an official source at the ministry as saying. The Pentagon expects to provide information on these matters tomorrow morning.
However, an official from the Assad regime has since boasted the strikes will have had little affect. The White House has reportedly been working with allies in France and the United Kingdom over the last week to coordinate a response to the chemical attack in Douma.
"The United States responded with 58 missile strikes that destroyed 20 per cent of the Syrian Air Force". Although Assad's government denied responsibility, Macron said he has "no doubt" that the Syrian regime was behind the attack.
"The fact of this attack should surprise no-one", British Prime Minister Theresa May added in a statement.
More: Russia just blamed Britain for Syria chemical attack.
Allegations of Assad's chlorine use are frequent in Syria's conflict, raising questions about whether Washington had effectively lowered the threshold for military intervention.