MPs would need to approve British military action in Syria.
Legally, the government doesn't need the permission of Parliament to use military force. But there is a convention that it will consult MPs before doing so, where possible, and abide by the outcome of their vote.
"Britain is not taking part in the action in Syria because MPs have so far refused to authorise the extension of airstrikes in neighbouring Iraq."
Mail Online, 16 November 2015
"I need to take it to Parliament"
David Cameron, 16 November 2015
In the aftermath of the atrocity in Paris, there's been a renewed focus on UK military action against Islamic State in Syria. It's claimed that this would need parliamentary approval.
That may be so politically, but not legally. Strictly speaking, as the House of Lords Constitution Committee has put it:
"The decision to engage in armed conflict is one for the Government; Parliament has no legal role in authorising or approving the use of the armed forces overseas".
What about MPs, though? If there's no legal reason to consult them, why does the Prime Minister say he needs to win a vote before taking the military action he wants?
We can do no better than point to the comprehensive briefing by the House of Commons Library on this subject.
It notes that the Labour government held a vote on sending troops to Iraq in 2003, even though there was no formal requirement for it. That led to debate in subsequent years about creating a more direct role for MPs in authorising military force.
The Coalition government said in 2011 that it accepted that there was, by then, a constitutional convention:
"before troops are committed, the House should have an opportunity to debate the matter. We propose to observe that convention except when there is an emergency".
One definition (there are many) of constitutional conventions is
"practices which are politically binding on all involved, but not legally binding".
But they aren't an exact science. The government made the commitment above on 11 March 2011. On 19 March, the UK began military operations in Libya. The House of Commons didn't vote on the matter until 21 March.
Professor Gavin Phillipson argues that the convention to consult MPs before taking military action wasn't honoured in this case; the authors of Constitutional and Administrative Law, by contrast, use the vote as an example of the convention in action.
Another complication that might have applied in the Libyan case is that the convention excludes "emergency" situations where there isn't time to get parliamentary approval.
And as deploying military force can take many forms, in some cases the convention mightn't be triggered. The government argued that there was no vote necessary when troops were sent to Mali in 2013, for instance, as they weren't in a "combat role".
But whatever about the details, there's no real doubt now that the convention exists and would cover something like air strikes in Syria. It's mentioned in the Cabinet Manual—the official guide to how the country is run.
And in 2013, when MPs voted against intervening in Syria, the government dropped the idea.
Arguably, then, the convention now requires not just that MPs be consulted on the use of armed force, but that they vote for it. A vote against it "would make military action impossible, at least until circumstances changed".
This is still speaking politically, rather than legally: a constitutional convention is just that, a settled political understanding. It can be broken.
Some Conservative politicians hint that they'd like to break this one. Dr Liam Fox, a former defence secretary, reportedly said yesterday that waiting for parliamentary approval isn't "constitutionally correct. I don't believe it's necessary, but it seems to be the convention that we have adopted".
As the constitutional expert Peter Hennessy puts it, these conventions are fragile: "they can crumble at the touch of a powerful, insensitive and determined" government.
So while it makes sense for the Prime Minister to build public and political support for intervention in Syria, he is entitled to act without it.