The Prime Minister accepted that, after Iraq, the public was sceptical and war-weary but insisted the situation in Syria was not the same.
"It's not about taking sides in the conflict; it's not about invading. It's not about regime change or indeed working more closely with the Opposition; it's about the large-scale use of chemical weapons and our response to a war crime - nothing else."
He said it was "beyond doubt" the Syrian regime had used chemical weapons and that the evidence about the latest attack, including at least 95 videos, was "right in front of our eyes".
Mr Cameron insisted any military action would first have to be approved by another Commons vote and have to be "legal, proportionate and specific" in deterring the further use of chemical weapons.
He accepted there was no one single smoking piece of evidence and that ultimately it was a judgment call but he stressed it was in Britain's national interest that the ban on chemical weapons was upheld.
The PM accepted there was "no 100% certainty about what action might succeed or fail" but warned, given the Assad regime had used chemical weapons, that "if nothing is done, it will conclude it can use these weapons again and again and on a larger scale, and with impunity".
Labour leader Ed Miliband, whose amendment called for a delay until the United Nations inspectors reported, stressed that the UN was "not some inconvenient sideshow" but was vital before any military intervention could took place.
The Labour leader, to murmurs of disbelief from Tory benches, insisted he did not rule out military action but emphasised how any plans had to be carefully scrutinised on the basis of the consequences they would have.
Claiming Mr Cameron had to make a better case for intervention, he told the PM: "He cannot say this does not change our stance on Syria, this does not change our stance on the Syrian conflict. Frankly, it would."
Sir Malcolm Rifkind, the former Conservative Foreign Secretary, warned that any failure to act would send the wrong message to Damascus.
"There is no guarantee that a military strike against military targets will work but there is every certainty that, if we don't make that effort to punish and deter, then these actions will indeed continue," he said.
Fellow Tory Liam Fox the former Defence Secretary, said that, while he understood widespread concern about military intervention in Syria, he believed that to do nothing would be an "abdication of our international, legal and moral obligations".
He added: "To do nothing would be interpreted in Damascus as appeasement of a dreadful regime and of the dreadful actions it has carried out. Appeasement has never worked to further the cause of peace in the past.
"It will not now and it will not in the future."
But Angus Robertson for the SNP spoke out against swift military action, insisting that diplomatic and humanitarian efforts should be the focus of the international community.
"Having been misled on reasons for war in Iraq … the least the UK Government could have done is provided detailed evidence; frankly, it hasn't," he told MPs.
George Galloway, the Respect MP, also warned against UK military action, claiming the Syrian rebels definitely had sarin gas and "you don't have to be Einstein" to carry out a chemical attack.
"Russia and China say no to war. So do I and most people in this country," declared the Scot.
Jack Straw, who was Foreign Secretary when the UK joined the US in attacking Iraq in 2003, noted: "We all know - I have the scars about this - how easy it is to get into military action and how difficult it is to get out of it."
In the Lords, Lord Howard, the ex-Conservative leader, argued that Britain must not be paralysed by the Iraq war and risk being humiliated as the US and France intervened in Syria.
But Lord Forsyth, the former Scottish Secretary, said it defied common sense that a military strike on Syria would not lead to the radicalisation of Muslims.
He said pushing ahead with military action would be an act of supreme folly.
Lord Dannatt, former head of the British Army, said he, too, did not support military intervention at this time.
The crossbench peer argued a campaign plan must have a beginning, a middle and an end plus an exit strategy that left Syria in a better position.
He said: "I don't think we know how to do that because the risks, the uncertainties and the unintended consequences are too great."