In April, the U.S. military attacked a Syria-government airfield with 59 Tomahawk missiles. It was the first direct military action the U.S. had taken against the regime of Syrian leader Bashar al-Assad.
As Trump officially announced the strike, he said the targeted airfield had launched the chemical attack on a rebel-held area and called upon other nations to oppose Syria's embattled leader.
In response, Russia said the strikes violated international law. Moscow also warned the move would cause significant damage to relations between Russia and the U.S.
Russia has been unwavering in its support of Syrian President Bashar Assad, who has been accused of war crimes and the use of chemical weapons against his own people. The U.S., meanwhile, has provided arms and training to anti-government rebel groups.
Human rights activists have urged the UN to charge Russia and Iran, allies of the Syrian regime, with war crimes after thousands of Russian airstrikes were reported to have killed more than 4,000 Syrian civilians. U.S.-led international coalition airstrikes targeting the Islamic State are also reported by rights groups to have killed more than 1,000 civilians.
— CNBC's Nyshka Chandran and Natasha Turak contributed to this report.