What's unclear is where that red line now stands.
The UN Security Council has failed to adopt Russia's draft resolution condemning the recent joint airstrikes by the US, Britain and France on Syria, which came following an alleged chemical weapons attack on the Syrian town of Douma.
Saudi Arabia, which along with other Gulf states has been a key backer of Syrian opposition groups fighting Mr Assad, said that it "fully supports the strikes launched by the United States, France and Britain against Syria because they represent a response to the regime's crimes".
"A perfectly executed strike last night".
After failing to talk the U.S. out of bombing targets in Syria, Russia responded to Friday's strikes by saying they "threatened" Russia and warning of "consequences".
In last year's strikes, the Pentagon said missiles took out almost 20 percent of the Syrian air force. That makes degrading it through military strikes far more hard.
While acknowledging that Assad may retain some chemical weapons capabilities, the Pentagon proclaimed that the action struck at "the heart" of Assad's program.
It claimed that Washington, encouraged by the Israeli and Saudi governments, was plunging the region into war, and asserted that the solution to the Syrian civil war, which has claimed more than 350,000 lives since 2011, is a "political" one.
"We are confident that we have crippled Syria's chemical weapons program".
Defense Secretary Jim Mattis said he was only certain about intelligence that chlorine gas was used by Assad's forces on April 7 before America's strikes on Syria.
He did not rule out sarin use.
US Ambassador Nikki Haley told an emergency Security Council meeting called by Russian Federation that she had spoken to President Donald Trump earlier about the military action in response to an alleged chemical attack a week ago.
While American President Donald Trump has called Friday's airstrikes against the Syrian military a success, members of that country's diaspora are calling the lack of follow-up a disappointment.
Speaking on the attack on Syria, British UN Envoy Karen Pierce has said: "It can not be illegal to use force to prevent the killing of such numbers of innocent people", she said in reference to the alleged chemical attack. It blamed Islamic State militants for mustard gas use.
The narrowly targeted pre-dawn US-French-British operation, which took aim at three chemical weapons facilities, earned quick scorn from Russia, But Moscow's push for condemnation of the strikes at the UN Security Council fell short. Moscow slammed the joint United Nations and OPCW inquiry as flawed. "All responsibility for them rests with Washington, London and Paris".
The United States, Britain and France have argued that military action was necessary after Assad's forces had used toxic gases multiple times in violation of worldwide law. White said she could not go into detail about that evidence.
The investigators, who are only mandated to determine if chemical weapons were used and not who used them, were expected to start their investigations into the Douma incident on Saturday, the Netherlands-based organization said.
Mr Trump said the combined operation would continue: "We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents".