And then on Friday night, the countries hit three targets - including one on the outskirts of Damascus - all related to Syria's chemical weapons program: a research center, a storage facility, and an equipment facility and command post.
Theresa May has said the air strikes sent a "clear message" against the use of chemical weapons.
"A combined operation with the armed forces of France and the United Kingdom is now underway", Trump said.
In another tweet minutes later, Trump added how he's "so proud" of the US Military, which will soon be "the finest that our Country has ever had...after the spending of billions of fully approved dollars". "There won't be anything, or anyone, even close!"
Missiles were launched from both the sea and aircraft.
Last April, Trump ordered a cruise missile strike against a Syrian airbase last April after a similar chemical weapons attack on civilians.
"The Arab Republic of Egypt expresses its solidarity with the brotherly Syrian people in their aspirations to live in security and stability", the ministry said in a statement.
Also not specifically mentioning Saturday's missile strikes, Egypt's foreign ministry said it was deeply concerned "at the current military escalation in Syria".
He also said there was no indication that Russian air-defense systems were employed during the attacks.
A Syrian soldier films the damage of the Syrian Scientific Research Centre.
The strikes significantly crippled Syrian President Bashar al-Assad's ability to produce chemical weapons, officials told reporters at a briefing, and the Pentagon was not aware of any civilian casualties resulting from the strikes. "There has been a 2,000 percent increase in Russian trolls in the last 24 hours". "Therefore, we will keep you all abreast of the facts moving forward".
The U.S. had no coordination or agreements with Russians before the strike, McKenzie said. But, the US believes, Assad's regime was still responsible for yet another attack on Sunday.
Russian President Vladimir Putin said his country would convene an emergency meeting of the UN Security Council to discuss the actions of the USA and its allies, while UN Secretary-General Antonio Guterres urged respect for the UN charter and global law.
The Pentagon repeated Saturday that the President has the authority under the Constitution to defend United States interests, but what happens next is up to the Assad regime and Russian Federation.
Gen. Joseph Dunford, Joint Chief of Staff, said the strikes were carried out on three Syrian chemical weapons infrastructures and the US believed it would send al-Assad a strong message. "The bombing maybe will also help in getting rid of the Assad regime and find a solution for the crisis by having new order that we the Syrian people want".
During simultaneous events in Washington last week, the president said he would make a decision "very quickly" on when U.S. troops would pull out of Syria while his top commander in the region - Army General Joseph Votel, head of U.S. Central Command - said he saw a continuing role for the U.S. military in helping to stabilize the divided country. Putin demanded a United Nations Security Council immediately.