Who carried out the chemical attack in Ghouta on August 21, 2013?

On August 21, 2013, rockets with chemical payloads landed around the neighborhood of Zamalka, located in Ghouta (an agricultural area outside of Damascus), resulting in hundreds of civilian casualties. A UN team visited the area and confirmed that the rockets carried sarin, a deadly nerve agent used as a chemical weapon. There was initial skepticism that the Syrian government would have ordered such an attack, risking retaliation by the US. But information published by the US, UN, and Human Rights Watch during the following weeks convinced Western public opinion that the Syrian government was indeed responsible. This information included: 1--Proof that the volcano rockets found in Zamalka were locally manufactured by the government. 2--Rocket trajectories reported by the UN that were shown to intersect at a Syrian army base. 3--Detection of sarin, which was known to be held by the Syrian government. 4--Reports by the US government that the rockets had been launched from government territory. 5--Finally, Syria’s agreement to dismantle their chemical weapons program was further seen as an admission of guilt. At that point the issue mostly disappeared from public discourse. However, over time, some of this evidence has been found to be unreliable, while new evidence has surfaced that seems to better support the hypothesis that opposition forces were behind the attack. These include: 1--Analysis of the rocket’s range showing that it could not have been launched from the army base initially suspected as the origin. 2--Errors in the UN trajectory calculations that, together with the new rocket range calculation, point to a launch from opposition territory. 3--Evidence of multiple instances in which the opposition used sarin or tried to acquire it. 4--Videos apparently showing an opposition group (Liwa al-Islam) launching chemical rockets, which match the time and location of the attack on Ghouta. This attack, and others that followed, are the subject of heated international debate, which significantly increased the risk of a US-Russian military conflict on at least three occasions. The world clearly needs an unbiased analysis of this case using advanced inference tools.

Update: In June 2021, the videos of the event were corroborated as authentic. They were matched to a field within opposition controlled territory, and that field has been shown to be in the intersection of seven trajectories calculated from images of the impact sites. See additional evidence.

We now consider this issue closed, and no longer see a need for a probabilistic analysis, as the new discoveries are clear enough to understand using standard human reasoning. We're leaving the analysis here for reference, and recording our original estimate: 87% Opposition, 13% Syrian army.

Note: While we don’t expect anyone to apply at this point, the Rootclaim $100,000 debate challenge is still available for this issue. Read more here.