A bellicose President Trump wanted to kill Syrian dictator Bashar al-Assad after he launched a chemical weapons attack on civilians in April 2017, according to the new White House tell-all by Bob Woodward of the Washington Post.

The commander-in-chief called Defense Secretary Jim Mattis and issued the deadly order.

“Let’s f–king kill him! Let’s go in. Let’s kill the f–king lot of them,” Trump said, according to the book, the subject of a lengthy article in the DC paper on Tuesday.

Mattis told the president that he’d get right on it.

But after hanging up, he told a senior aide: “We’re not going to do any of that. We’re going to be much more measured.”

The president’s national security team then prepared a plan for a conventional airstrike that Trump ultimately ordered.

Trump tweeted a day later that the military attack against Syrian targets was a “Mission Accomplished!”

“A perfectly executed strike last night. Thank you to France and the United Kingdom for their wisdom and the power of their fine Military. Could not have had a better result,” Trump wrote.

But the Syrian civil war still grinds on after seven years of fighting — and Assad, along with his Russian and Iranian allies, reportedly is prepping for a massive assault on Idlib Province, the rebels’ last stronghold, which observers fear could kill hundreds of thousands of civilians.

Trump warned the Syrian strongman in a tweet Monday to back off.

“President Bashar al-Assad of Syria must not recklessly attack Idlib Province. The Russians and Iranians would be making a grave humanitarian mistake to take part in this potential human tragedy. Hundreds of thousands of people could be killed. Don’t let that happen!” he wrote.