In April, Trump ordered an airstrike on Syria's Shayrat airbase, which was hit with 59 Tomahawk cruise missiles in, following an alleged chemical attack in Idlib earlier that week that was blamed on the Assad government.
The US has taken a series of actions over the past three months demonstrating its willingness to carry out strikes, mostly in self-defence, against Syrian government forces and their backers, including Iran.
At the time, United States officials said the intervention was a "one-off" meant to deter future chemical weapons attacks and not an expansion of America's role in the Syrian war.
It warned the use of chemical weapons "would likely result in mass murder of civilians, including innocent children", adding that Assad and his military would "pay a heavy price".More news: Centre gives green signal for Greater Noida International Airport
Trump said at the time that the chemical attack crossed "many, many lines", and called on "all civilized nations" to join the U.S.in seeking an end to the carnage in Syria.
Washington has repeatedly struck Iranian-backed militia and even shot down a drone threatening US-led coalition forces since the April military strike.
In April, global chemical weapons inspectors found what they called "incontrovertible" evidence that Sarin gas, or a similar substance, was used in the chemical attack that killed 89 people.
In the days following, Defense Secretary Jim Mattis cautioned that the US was prepared to take further action if Syria repeated such chemical weapons use.
The Syrian regime was widely blamed for the sarin gas attack in Khan Sheikhoun on 4 April, but it denied responsibility.More news: Home sales up in May, but buyers face sharply rising prices
The Kremlin also dismissed the White House statement, which had warned that Assad and his military would "pay a heavy price" if it goes ahead with the attack. As we reported, the missiles targeted aircraft and aircraft shelters, ammunition, air defense systems and radars.
"The US is preparing a new attack on the positions of the Syrian troops, this is clear".
Syria's war began in March 2011 with anti-government protests that spiralled into a complex and devastating conflict that has killed more than 320,000 people. Russian Federation is a close ally of Assad.
Tensions have heightened between key players - especially between the U.S. and Russian Federation - over Syria as it has transformed from a civil war to a complex battle involving global players and their proxies.More news: Fed: Biggest US banks strong enough to withstand recession