Let's take a look at how this latest crisis developed since the weekend.
With the Syrian army on the verge of recapturing Douma on Saturday, Islamist rebels claimed government forces had dropped a combination of chlorine and an unnamed nerve agent killing dozens of people. Syria and Russia insisted no such attack had taken place.
On Sunday, US President Donald Trump threatened a military attack on Syria, saying there would be a "big price to pay". The rebels then agreed to evacuate Douma within 48 hours and release several hundred hostages.
On Monday, Russia said Israeli warplanes had fired eight missiles from Lebanese airspace at an airbase in the central Syrian province of Homs. Russia said eight of the missiles were intercepted. The attack reportedly killed 14 people, including four Iranian soldiers.
On Tuesday, Russia said it had visited hospitals in Douma and found no evidence of any chemical attack.
On Wednesday, Trump continued to threaten imminent military action as Russian troops deployed to Douma.