Just over a year ago, Parliament was recalled to approve UK military intervention in the civil war in Syria - and despite a progressive watering-down of the proposition put before MPs, to the point where it became a kind of paving motion, with a further vote to be held on actual use of force - the government was dealt an unprecedented rebuff.
"We get it," a rueful David Cameron told MPs, and Friday's sitting is the direct result; prime ministers now need the approval of Parliament in these circumstances - or at any rate the approval of the Commons.
Tellingly, MPs will debate a substantive motion to authorise military action, moved by the prime minister, while the Lords will have a "take note" motion (and on the evolving Speakers' List for the Lords debate, there are 53 names and counting...) so what will really matter on Friday will not be the nuanced reservations of former foreign secretaries and diplomats on the red benches, but the numbers trooping through the lobbies in the Commons.
After his 2013 experience, I'm sure the prime minister would not be recalling Parliament without a cast iron assurance that his (and Labour's) whips can deliver a resounding Commons vote in favour of intervention - and the horrors of IS make that a far easier sell than the Syrian intervention proposed a year ago.
Even so expect some probing questions, and probably a number of dissenters. Tory MP John Baron, who in 2013 pushed through a motion requiring parliamentary approval for military action in Syria, said today his vote would depend on the answers to questions about the government's wider strategy. As I write, the detailed motions have not been published - but they will certainly attract plenty of scrutiny, when they emerge.
Already Nick Clegg - who will be winding up tomorrow's Commons debate - has said a further vote would be required if Britain was to take part in operations in neighbouring Syria, saying: "We'd go back to Parliament for another debate and a vote; I think that's a sensible way to proceed."
Where once a prime minister, armed with the Royal Prerogative, the powers delegated from the Monarch, could commit Britain to military action without an explicit Commons vote we now have a precedent for the need to seek parliamentary approval (in this kind of case, at least; no-one imagines British forces under attack would have to wait on a Commons debate before being allowed to shoot back) and a precedent for that approval being withheld.
For a summary of the current position take a look at this helpful inquiry by the Commons Political and Constitutional Reform Select Committee, which is keen to get a solid set of rules onto the statute book before someone backtracks.
The motion to be debated by the Commons tomorrow has been agreed by the Cabinet and has just been published.
It is carefully worded and lays out the key points that the government regards IS a threat to the UK directly, that the government of Iraq has requested assistance and that the authorisation does not extend to action in Syria and that a further vote would be required for that.
Here is the full text:
"That this House:
Condemns the barbaric acts of ISIL against the peoples of Iraq including the Sunni, Shia, Kurds, Christians and Yazidi and the humanitarian crisis this is causing;
Recognizes the clear threat ISIL pose to the territorial integrity of Iraq and the request from the Government of Iraq for military support from the international community and the specific request to the UK Government for such support;
Further recognizes the threat ISIL poses to wider international security and the UK directly through its sponsorship of terrorist attacks and its murder of a British hostage;
Acknowledges the broad coalition contributing to military support of the Government of Iraq, including countries throughout the Middle East;
Further acknowledges the request of the Government of Iraq for international support to defend itself against the threat ISIL poses to Iraq and its citizens, and the clear legal basis that this provides for action in Iraq;
Notes that this motion does not endorse UK air strikes in Syria as part of this campaign, and any proposal to do so would be subject to a separate vote in Parliament;
Accordingly supports Her Majesty's Government, working with allies, in supporting the Government of Iraq in protecting civilians and restoring its territorial integrity, including the use of UK air strikes to support Iraqi, including Kurdish, security forces' efforts against ISIL in Iraq;
Notes that Her Majesty's Government will not deploy UK troops in ground combat operations;
Offers its wholehearted support to the men and women of Her Majesty's armed forces."