David Cameron insisted he had "learned the lessons of Iraq" as world leaders at the G8 summit in Northern Ireland set out an agreed blueprint for the future of Syria.
But the British Prime Minister was again warned not to consider arming the Syrian rebels by President Vladimir Putin, who compared them to the killers of Drummer Lee Rigby.
Cameron and President Barack Obama persuaded Putin to agree to plans for a transitional government that could eventually replace the Assad regime, after intensive private discussions.
The Prime Minister appealed directly to supporters of the Syrian dictator to turn against him, promising that they could have a role in a future Syrian government. They should "know in their hearts" that the Assad regime was doomed, he said.
The Prime Minister heralded the deal agreed on the final day of the G8 summit in Northern Ireland as a "strong statement" that will pave the way for a Syrian peace conference.
It was agreed with Putin unexpectedly after months of the Russian President propping up the Assad regime. Although yesterday's statement does not call for Bashar al-Assad to step aside, it does agree to work on a "transitional governing body ... formed by mutual consent" which British sources said amounted to a tacit acknowledgement that the dictator should be replaced.
Cameron said it was "unthinkable that President Assad can play any part in the future of his country", adding: "He has blood on his hands."
But within hours of the deal being signed, Putin again warned other G8 leaders against arming the Syrian rebels.
"Recently the British people suffered a huge loss," the Russian President said, referring to Rigby's killing.
Many members of the Syrian opposition were aligned with those who killed Rigby.