Prime Minister David Cameron has set out his case for British jets and drones to strike so-called Islamic State targets in Syria in an address to the Commons.
Mr Cameron said Islamic State posed a "serious and undeniable" threat to Britain and British citizens, and said it was in the UK's national interest to take action.
He said if the country chooses not to act, after the attack on "our close friends and allies" in France, Britain's allies "could be forgiven for asking if not now, when?"
MPs were then given chance to pose questions on what he had to say.
Islamic State militants have been behind more than 40 terrorist attacks around the world in the past 12 months, Mr Cameron said, including high-profile incidents such as:
130 people killed in Paris earlier this month
43 people killed in an attack in Beirut earlier this month
102 people killed at a peace rally in Ankara in October
224 people killed when a Russian passenger jet was bombed over Egypt, also in October
38 people killed in at attack at a holiday resort in Tunisia in June, including 30 British citizens
During his speech, the PM insisted there would be no Western boots on the ground, arguing this would be "counter-productive" and may assist the extremists in recruiting more jihadists.
He said there was not a "purely military solution" to the situation in Syria, but said airstrikes would be the first step in working towards a political and diplomatic resolution.
He went through his answers to each of the seven points for the Government to answer before asking the House of Commons to approve military action in Syria, as posed by the Foreign Affairs Select Committee in a report released last month.
Mr Cameron had answered the questions more fully in a letter to the Commons Foreign Affairs Committee released ahead of his speech.
The seven key questions:
How would the proposal for an international strategy improve the chances of success of the international coalition's campaign against ISIL?
Without action against Islamic State's capital in Raqqa, Syria all progress against the terror group in Iraq is lost, the Prime Minister said.
The UK's world-leading military capabilities could contribute significantly to the operation, he said. In addition, the demonstration of international unity against the group "sends a powerful message to ISIL, and those who might be drawn to its poisonous ideology", Mr Cameron added.
How would the proposed action contribute to the formation and agreement of a transition plan for Syria?
Defeating Islamic State would, Mr Cameron said, help strengthen the moderate opposition in Syria as well as strengthen the country's internationally-recognised borders.
This is, he told the Commons, an "ISIL-first strategy" - adding that while the ultimate goal is to remove Assad from governance, there must be a clear sequence of events.
In the absence of a UN Security Council Resolution, how would the Government address the political, legal, and military risks arising from not having such a resolution?
UN Security Council Resolution 2249 was passed on November 20, stating that IS "constitutes a global and unprecedented threat to international peace and security", and calling upon member states to take "all necessary measures... to prevent and suppress terrorist acts committed specifically by ISIL... and to eradicate the safe haven they have established over significant parts of Iraq and Syria".
Does the proposed action have the agreement of the key regional players (Turkey; Iran; Saudi Arabia; Iraq) - and if not, would the Government seek this before any intervention?
Saudi Arabia, Turkey, Iraq, Jordan, the United Arab Emirates, Lebanon, Kuwait, Qatar, Egypt, Oman, Bahrain and Morocco are all members of the coalition already carrying out airstrikes in Syria.
"Both Turkey (which faces an acute threat from ISIL in Northern Syria) and Jordan (which faces particular threats from ISIL in Southern Syria and from Western Iraq) have made clear that they would welcome the UK joining the Coalition's strikes against ISIL in Syria," the PM's letter stated.
Which ground forces will take, hold, and administer territories captured from ISIL in Syria?
While land regained from IS in Iraq can be handed back to Iraqi security forces and the Kurdish Peshmerga, the Assad regime in Syria means the situation is more complex.
However, the PM added there are around 70,000 moderate Syrian opposition fighters who, along with Syrian Kurds, are able to administer leadership as well as defending their territory.
What is the overall objective of the military campaign?; does it expect it will be a "war-winning" campaign?; and if so, who would provide war-winning capabilities for the forces?; and what does the Government expect will be the result of extending airstrikes to Syria?
The overall objective given by Mr Cameron is to "degrade ISIL's capabilities so that it no longer presents a significant terrorist threat to the UK or an existential threat to Iraq, Syria or other states".
He said this incorporates the UK's counter-terrorism strategy while negotiating a political settlement for Syria, continuing humanitarian work, and trying to mitigate the impact of the group on the stability of the Middle East overall.
What extra capacity would the UK contribute to the coalition's actions in Syria?
The UK has "advanced military capabilities" which put the country's abilities above those deployed by most other coalition powers, Mr Cameron said.
This includes "dynamic targeting" by the RAF, where pilots are able to provide rapid support, with Brimstone missiles which strike "accurately" with "low collateral damage".
Even the US does not posses that capability, Mr Cameron said.
Labour leader Jeremy Corbyn asked a number of follow-up questions.
He began by condemning the actions of IS, saying the group was "contrary to everything the people on these benches have struggled for for many generations", and said there was "no doubt" the militants posed a threat to British citizens.
His questions included:
Does the Prime Minister believe that extending airstrikes to Syria - which is already being bombed by the United States, France and Russia, and other powers - will make a significant military impact on the ground, which has so far seen ISIL gain as well as lose territory?
Can he rule out the deployment of British ground forces to Syria?
Does he believe that more military force over Syria could increase dangerous incidents such as the shooting down of a Russian military aircraft by Turkish forces this week?
How could it contribute to the political resolution of the situation in Syria?
What advice has he been given about the likely impact of British air strikes in Syria on the threat of terrorist attacks in Britain?
What impact could an intensified air campaign have on civilian casualties in Syria and the surrounding areas?
The SNP also voiced its opposition.
The party's Westminster leader Angus Robertson said a number of questions remained unanswered, including over the lack of a plan for reconstruction in the country following a bombing campaign such as the one proposed.
Unless these questions are answered, he said, the SNP would not be voting in favour of airstrikes.