President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE reportedly wanted to assassinate Syrian President Bashar Assad after a chemical attack on Syrian civilians in April 2017.

“Let’s f---ing kill him! Let’s go in. Let’s kill the f---ing lot of them,” Trump told Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE, according to a soon-to-be-released book by journalist Bob Woodward.

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Mattis responded to the president that he would begin working on plans, but then told an aide, “We’re not going to do any of that. We’re going to be much more measured.”

The incidents are described in excerpts published Tuesday by The Washington Post.

The Pentagon, instead, convened plans for a more conventional airstrike targeting Syrian military instillations, which were ultimately carried out.

Trump warned Assad against a potential upcoming offensive, tweeting, “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!”

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! — Donald J. Trump (@realDonaldTrump) September 3, 2018

The U.S. has provided material support for various rebel groups in Syria in their fight against Syria and its Russian and Iranian allies.

The Syrian civil war has killed hundreds of thousands of civilians and forced millions to seek refuge inside and outside Syria.