In a speech to the United Nations, Cameron said the international community must not be "so frozen with fear" by old mistakes that it withdrew into "isolation" and failed to confront the threat of Isis.
The Prime Minister later told the UN Security Council: "We need to act and we need to act now."
MPs will be recalled to give their approval for British air strikes tonight.
Cameron discussed the plan yesterday with Labour leader Ed Miliband, who will support the House of Commons motion. The Cabinet were to meet today.
The move will mark the first time British combat forces have played an active role in Iraq since they withdrew five years ago. However there will be no UK involvement in air strikes in Syria and no "boots on the ground".
The decision followed an official request for British help by Iraqi Prime Minister Haider al-Abadi. Cameron also met Hassan Rouhani, the President of Iran, yesterday in an effort to build support for air strikes across the Middle East. It was the first face-to-face meeting with an Iranian leader since 1979.
Addressing the UN General Assembly, Cameron said that while it was "absolutely right" that Britain and the West should "learn the lessons of the past", those lessons had to be the "right lessons".
"Yes to careful preparation; no to rushing to join a conflict without a clear plan. But we must not be so frozen with fear that we don't do anything at all. Isolation and withdrawing from a problem like [Isis] will only make things worse. We must not allow past mistakes to become an excuse for indifference or inaction. The right lesson is that we should act - but act differently.
"We should be comprehensive defeating the ideology of extremism that is the root cause of this terrorism so we win the battle of ideas and not just the battle of military might."
He claimed Isis was a "clear and present danger" to the UK. "I think this is completely different to 2003. I think everyone can see that [Isis] is a direct threat to the UK. They have taken hostages, they have conducted plots to kill and maim British citizens."
Miliband also voiced support for action against Isis. "We cannot turn away from the threat of [Isis] which is a murderous organisation, has taken British hostages, threatens the stability of the region and is therefore a threat to the UK's national interest. That is why we will be supporting the Government's proposal for UK air strikes in Iraq against [Isis]. I want to reassure people there is no question of committing UK ground troops."
Alongside US aviation and cruise missiles, Saudi Arabia, Bahrain, the United Arab Emirates and Jordan have hit targets in Syria and France in Iraq. France's Defence Minister said last night that the country is considering whether to extend airstrikes to Syria. Qatar is providing logistical support. Belgium and the Netherlands have committed six warplanes each to Iraq with several hundred military personnel.
US President Barack Obama issued a rallying call for the world to unite behind the military campaign to destroy Isis' "network of death". Speaking at the UN, he outlined in dire terms the "generational" struggle to combat the "the cancer of violent extremism".
Obama later chaired an unusual session of the UN security council on foreign jihadists which adopted a resolution requiring countries to pass laws to prosecute citizens who join jihadist groups or send them funds.
He said the world stood at a crossroads between war and peace and spoke of the "pervasive unease" across the planet in the face of challenges ranging from the Ebola epidemic to Russian aggression in Ukraine.
He directed much of his speech at the Islamic world as he urged political and religious leaders and individual Muslims to take on the extremists everywhere from the battlefield to social media. "No God condones this terror," he said.
"No grievance justifies these actions. There can be no reasoning - no negotiation - with this brand of evil. The only language understood by killers like this is the language of force. So the United States of America will work with a broad coalition to dismantle this network of death.
-Telegraph Group Ltd, Independent, AFP, AP
