The Commons was the scene of one of the greatest foreign policy upsets in living memory today, after MPs voted against the government motion on military intervention in Syria.
The vote, which was lost by 272 to 285, brings to a close any hope of Britain participating in any military action against the regime of president Bashar al-Assad but it also serves to dramatically weaken the prime minister in the eyes of parliament and his party.
Amid shouts of 'resign' in the Commons chamber, Cameron said: "The British parliament does not want to see military action. I get that and the government will act accordingly."
The only possible comparison of a party leader not being able to command the support of his own side in a matter of foreign policy is Neville Chamberlain in the Norway debate of 1940.
Back then, Labour called a vote which effectively functioned as a vote of confidence in his leadership.
When a quarter of Chamberlain's MPs voted with the opposition he resigned and Winston Churchill became prime minister.
It is highly unlikely that today's vote would face Cameron from power, but it is a brutal humiliation for a prime minister who clearly overplayed his hand on the Syria crisis.
After a gruelling seven-and-a-half hour debate, dozens of MPs on all sides of the House expressed severe reservations about becoming embroiled in the civil war in Syria.
The vote came after a Labour amendment to the government motion on intervention in Syria was been shot down by MPs by 332 votes by 220.
Labour's junior shadow transport spokesman, Jim Fitzpatrick, resigned during the course of the debate as he felt even the Labour amendment, which had more safeguards than the government motion, was too open to being used for military action.
Behind the scenes, Labour and Downing Street dramatically ended their truce during the debate, after No 10 branded Ed Miliband "incoherent".
As debate raged in the Commons, the prime minister's spokesperson said the Labour leader was "flipping and flopping" after he refused to back the government's motion on military intervention and instead moved his own amendment.
Miliband was accused of being "unable to make up his mind" after he reportedly said he would back the government in an evening meeting yesterday. That claim was immediately denied by Labour.
No 10 said Miliband had changed the" goalposts" of intervention "dramatically and significantly" yesterday evening and accused him of "playing politics" with issues of war and peace.
Labour sources reacted angrily to the comments from No 10, saying they were "uncalled for" and "demeaning to the debate".
They said it was "categorically untrue" that Miliband had offered the prime minister assurances only to go back on them hours later.
Downing Street's assertion that a divided House of Commons would provide "succour" to President Assad was branded "frankly insulting".
The developments seemed to be reflected by the body language of David Cameron and Miliband in the Commons. The prime minister could barely bring himself to look at the leader of the opposition as they debated military intervention.