Peter Ford, who served in Damascus from 2003 to 2006, said it was out of character for the Syrian president to provoke his US counterpart, in this case Donald Trump, just as Washington was taking a softer line compared to Barack Obama's policies on Syria.
On Friday, US missiles hit a Syrian airfield in retaliation for an attack in Khan Sheikhun with a Sarin-like nerve agent that left up to 100 people dead. Washington blames the chemical incident on Assad, a charge the Syrian government has denied.
Ford said: "Assad is not mad and would have known that when Donald Trump produced an olive branch in his direction, any use of chemical weapons would have been counter-productive."
In the weeks before the deadly attack, US officials said that to remove Assad from power was no longer a top priority.
"He is someone who doesn't leap without looking. He is trained as an ophthalmologist," said Ford, who said he regularly met Assad during his service in Damascus. "He has an analytical mind and knows that actions have consequences."
Ford has disputed widespread reports that the Syrian government carried out a chemical attack. Instead, he is inclined to believe that Syrian aircraft inadvertently bombed an armed group-controlled chemical warfare factory or stockpile.
He said there should be "no rush to judgment, no rush to war" because of the chemical incident in Khan Sheikhoun; instead, there needs to be a proper investigation into the facts of what happened before decisions were taken.