The Syrian president is accused of a number of war crimes, including massacring his own citizens and ordering the torture of foreigners.
"I would have rather taken him (Assad) out. I had him all set. Mattis didn't want to do it, Trump told Fox News in an interview on Tuesday when asked if Assad had ordered chemical bombs be dropped on the north Syria town of Khan Sheikhoun in April 2017.
According to intelligence reports, the chemical agent sarin had been used, killing 86 people including 27 children.
"Do you regret not taking out Assad and was it because of Russia being allied with them (Assad) the Fox News presenter asked Trump.
"No, I do not regret that. I could have lived either way with that. I considered him certainly not a good person, but I had a shot to take him out if I wanted and Mattis was against it," Trump said.
The US president was critical of Mattis. "Mattis was against most of that stuff (the assassination bid). He'd keep you in military, but he didn't know how to win. He had no concept to win, Trump said.
Trump said they have made tremendous progress in the war against terrorism.
"I got rid of the Islamic State (IS) after he (Mattis) was gone. I did a great job on IS, 100 per cent of the caliphate (is gone). Got rid of them. Took out (Iranian major general Qasem) Soleimani as you know. Took out (Iraqi terrorist Abu Bakr) al-Baghdadi. These are the two biggest terrorists. Al-Baghdadi founded ISIS, he was trying to build it again after I wiped it out, he said.
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