Asked by reporters at a farewell press conference in Washington if the “red line” episode was a nadir of the modern presidency, Kerry argued that the media had not properly analysed, assessed or reported exactly what took place.

“The president of the United States of America, Barack Obama, did decide to use force and he announced his decision publicly and he said we’re going to act, we’re going to do what we need to do to respond to this blatant violation of international law and of warnings and of the red line he had chosen,” he said.

“Now, we were marching towards that time when, lo and behold … Prime Minister David Cameron went to the parliament … and he sought a vote for approval for him to join in the action that we were going to engage in. And guess what, the parliament voted no, they shot him down.”

MPs voted by 285-272 to rule out joining US-led strikes, triggering diplomatic shockwaves across the Atlantic.

In his unusually frank account, Kerry recalled: “So as we were briefing Congress – and I was on one of those briefing calls with maybe a hundred members of Congress on the call – many of them were saying, ‘Well, you’ve got to come to us. You’ve got to go through the constitutional process, get permission from us to do something.’

“And the president had already decided to use force but then the question became, ‘Do I need to go to Congress to get that permission?’

“It was a big debate in the security group. I was part of that, I remember the debate. And we felt that we’d quickly get Congress’s approval because this was such a blatant violation.

“I got a call Friday night, we met Saturday morning and the president decided that he needed to go to Congress because of what had happened in Great Britain and because he needed the approval and that was the way we’d do something like that.”

In the meantime, Kerry said, he held a press conference in London and was asked if there was anything Assad could do to avoid being bombed. He replied yes, he could agree to get rid of his weapons, and within an hour and a half Kerry received a phone call from Russian counterpart Sergey Lavrov suggesting they co-operate on such a deal – something Obama and Vladimir Putin had previously discussed.

“All of a sudden Lavrov and I were thrown together by our presidents,” Kerry said, “in an effort to try and achieve that and guess what, we did achieve it, before Congress voted.

“The president never said, ‘I won’t drop a bomb.’ What happened was people interpreted it. The perception was that he was trying to find a different road. And I will acknowledge to you, absolutely, I heard it all over the place. The perception hurt, yes.”

He added: “The perception came about despite the fact that we actually got a far better result of getting all of the weapons of mass destruction out of Syria without dropping a bomb. If we had dropped a bomb, there is no guarantee we would have got any of them out.”

Despite the removal of most of the Syrian chemical arsenal, Assad gained strength with Russian and Iran support. The civil war worsened – its death toll now approaches half a million and a huge refugee crisis continues. Obama’s fateful act of omission is much debated as he nears the end of his presidency.