Everybody, but everybody's convinced that Asad is responsible for what's just happened in Khan Sheikhoun, and that's because everybody, but everybody's forgotten about (or has chosen to ignore), what really happened in similar circumstances in Ghouta in 2013, and why Obama didn't go off half-cocked then as Trump has now with his cruise missile strikes on Syria's Shayrat airbase.
As veteran US journalist Seymour Hersh explained in his April 2014 essay, The Red Line & the Rat Line:
"In 2011 Barack Obama led an allied military intervention in Libya without consulting the US Congress. Last August , after the sarin gas attack on the Damascus suburb of Ghouta, he was ready to launch an allied air strike, this time to punish the Syrian government for allegedly crossing the 'red line' he had set in 2012 on the use of chemical weapons. Then with less than 2 days to go before the planned strike, he announced that he would seek congressional approval for the intervention. The strike was postponed... and subsequently cancelled when Obama accepted Assad's offer to relinquish his chemical arsenal in a deal brokered by Russia. Why did Obama delay and then relent on Syria when he was not shy about rushing into Libya? The answer lies in a clash between those in the administration who were committed to enforcing the red line, and military leaders who thought that going to war was both unjustified and potentially disastrous.
"Obama's change of mind had its origins in Porton Down, the defence laboratory in Wiltshire. British intelligence had obtained a sample of the sarin used in the 21 August attack and analysis demonstrated that the gas used didn't match the batches known to exist in the Syrian army's chemical weapons arsenal. The message that the case against Syria wouldn't hold up was quickly relayed to the US joint chiefs of staff. The British report heightened doubts inside the Pentagon. The joint chiefs were already preparing to warn Obama that his plans for a far-reaching bomb and missile attack on Syria's infrastructure could lead to a wider war in the Middle East. As a consequence the American officers delivered a last-minute caution to the president, which, in their view, eventually led to his cancelling the attack.
"For months there had been acute concern among senior military leaders and the intelligence community about the role in the war of Syria's neighbors, especially Turkey. Prime Minister Recep Erdogan was known to be supporting the Nusra Front, a jihadist faction among the rebel opposition, as well as other Islamist rebel groups. 'We know there were some in the Turkish government,' a former senior US intelligence official, who has access to current intelligence, told me, 'who believed they could get Assad's nuts in a vice by dabbling with a sarin gas attack inside Syria - and forcing Obama to make good on his red line threat'." (lrb.co.uk)
See also my 4/8/14 post About That Gas Attack in Syria... (Just click on the Seymour Hersh label below.)