Amidst a Syrian government offensive in Eastern Ghouta, a suspected chemical attack was carried out in the Syrian city of Douma on 7 April 2018, with at least 70 people reported as killed. The Jaysh al-Islam rebel group controlled Douma, and they reported Syrian Army helicopters had dropped barrel bombs; this was also reported by several medical, monitoring, and activist groups, including the White Helmets. The bombs were suspected to be filled with chemical munitions such as chlorine gas and sarin. The World Health Organization said it received reports from partner agencies that some 500 people arrived at health facilities showing "signs and symptoms consistent with exposure to toxic chemicals."
As with previous incidents, France, the United Kingdom, the United States, and other nations accused the Assad regime of being responsible for the use of chemical weapons. Russia and Iran, the Syrian government's main allies, denied chemical weapons had been used, claiming it was a false flag operation. Russia has said video of the chemical attack was staged by members of the White Helmets organization. The Syrian Arab News Agency reported Saudi Arabian-backed Jaysh al-Islam rebel coalition was making "chemical attack fabrications in an exposed and failed attempt to obstruct advances by the Syrian Arab Army".
In May 2017, French President Emmanuel Macron said the use of chemical weapons in Syria would be a red line requiring immediate reprisal. France and the United States cited positive urine and blood samples collected as proof of chlorine being used in Douma.
In the early hours of 9 April 2018, an airstrike was conducted against Tiyas Military Airbase in Syria. The United States denied launching the airstrike, and an Israeli spokeswoman declined to comment. On 10 April, an emergency United Nations Security Council meeting was held where competing solutions were presented on how to handle the response to the alleged chemical attack; all were ultimately vetoed. By 11 April, each of the Western nations began to consider military action in Syria, seeking a "strong joint response."
On 11 April, the Syrian government said it had invited the Organization for the Prohibition of Chemical Weapons to investigate the sites of the alleged attacks. "Syria is keen on cooperating with the OPCW to uncover the truth behind the allegations that some western sides have been advertising to justify their aggressive intentions," said the state news agency SANA, quoting an official source in the Foreign Ministry. Russia denied chemical weapons were used, then alleged on 13 April that Britain had staged the event in order to provoke US airstrikes.
By 12 April, British Prime Minister Theresa May had ordered Royal Navy submarines in the Mediterranean to move within striking range of Syria in readiness to attack ground targets with cruise missiles by the end of the week. British military sources later told The Times that days before the bombings a British Astute-class submarine armed with Tomahawk missiles and approaching within firing range of Syrian military targets was chased by "one, and possibly two" Russian Kilo-class submarines from the Russian naval base at Tartus. The Russian submarines were supported by two frigates and an antisubmarine aircraft, while the British submarine was assisted by a US Navy P-8 Poseidon patrol aircraft. Ultimately no British submarine took part in the strikes.