David Cameron, still lobster-red from his holiday, repeatedly pointed to the controversial 2003 decision, saying the well of public opinion had been poisoned by it and accepting there was a war-weariness across Britain. But he insisted Syria was not Iraq.
MPs - now recalled from the corners of the globe to vote on whether there should be another vote - were sombre, exuding a poignant undercurrent of concern.
The PM looked and sounded emotional as he said the evidence of Assad's guilt in the poison gas attack was "right in front of our eyes", pointing to the horrific videos, which were the most sickening imaginable.
He told MPs: "You can never forget the sight of children's bodies stored in ice, young men and women gasping for air and suffering the most agonising deaths."
Mr Cameron accepted there was no smoking gun but that all the available evidence pointed to Syria's culpability in the chemical attack.
Nonetheless, he admitted that, ultimately, this was a judgment call.
"This is a judgment. We all have to reach a judgment about what happened and who was responsible," he declared.
Ed Miliband, speaking in a low, measured voice, argued for a "sequential roadmap" going through the United Nations, having the inspectors' report and acquiring compelling evidence the Assad regime was culpable. He warned against adhering to an artificial timetable set elsewhere; that is, in Washington.
When he mentioned how he was not ruling out military action, there was a rumble of disbelief on Government benches as Conservative MPs shifted uncomfortably in their seats.
The PM, who 48 hours before thought he had the Labour leader standing shoulder to shoulder, looked to the ceiling as Mr Miliband spoke of being clear-eyed about the implications of military strikes.
When Tory grandee Sir Malcolm Rifkind interjected that all Labour desired was in the Government motion, he urged Mr Miliband to support the PM so Parliament could speak with a united voice. His Conservative colleague Nadhim Zahawi went further, accusing Mr Miliband of seeking to "divide the House for political advantage" and asked: "What has happened to the national interest?".
Labour MPs shook their heads in disgust and their leader replied: "That intervention is not worthy of the honourable gentleman."
It seems clear that, if there is a second vote to sanction UK military action, Mr Cameron, in attempting to get majority support, will have his work cut out.