If all goes to schedule, the House of Commons in London and Berlin's Bundestag will vote through increased involvement over Syria's skies on the same day. The UK mission is set to stretch to airstrikes, despite dissent.
Now that Germany's cabinet has agreed to provide nonaggressive military support over Syria, getting the motion through the German parliament, or Bundestag, should be a doddle for the Christian Democrats and Social Democrats. After all, their grand coalition holds more than 500 of the 630 seats.
The vote in the UK's House of Commons on expanding Britain's existing military mission against the so-called "Islamic State" (IS) to Syrian soil always seemed more precarious - given the ruling Conservatives' wafer-thin majority. Prime Minister David Cameron had said he would not take the issue of airstrikes to the floor for a vote, unless he was assured of success. So, consider Mr. Cameron certain.
Royal Air Force jets are already striking IS targets in Iraq, but voting "yes" would still mean an expansion of the British military mission. Defense Secretary Michael Fallon told a parliamentary committee meeting on Tuesday that "we will definitely be adding to" the existing force of eight jets based in Cyprus. "We will be stepping up the number of sorties," he also said, without giving details on the extent of a possible expansion.
Corbyn allows conscience vote
The key to unlocking the parliamentary vote lay in Labour's new leader, Jeremy Corbyn, probably best known in the UK for his staunch anti-war positions until the Labour leadership campaign propelled the longstanding London backbencher for Islington North onto the national stage this summer.
Corbyn won't support the motion, indeed he stands to lead the dissenters' lines in the debate before Wednesday's vote. Some months ago, you might even have found Corbyn outside Labour or Conservative HQ on Tuesday evening, leading the UK movement "Stop the War" in last-minute demonstrations against the plans.
"When you bomb a town like Raqqa, where there are several hundred thousand people living there, who may or may not wish to be there - indeed, many are trying to escape from there - we're going to kill people. We're going to kill people in their homes by our bombs. I think we should be very careful about that," Corbyn appealed during an appearance on BBC radio on Tuesday.
Crucially, however, facing disagreement within his own shadow cabinet - including from Shadow Foreign Secretary Hilary Benn - Corbyn agreed on Mondayto allow his MPs a conscience vote,
rather than trying to whip them into line. Having defied the Labour whips more than 500 times as a backbencher himself, observers await Corbyn's first effort to whip MPs with interest; the leader conceded in his interview with Jeremy Vine on Tuesday that he probably couldn't have altered his critics' votes if he had tried.
With even a smattering of Labour MPs onside, buttressing against around a dozen potential Conservative dissenters, the government's case should survive parliament.
Dereliction of duty?
The Scottish National Party look set to vote unanimously against the strikes, calling on Cameron's government to allow more time before a vote, and to publish his motion before the debate to allow proper scrutiny.
"With the Labour Party giving up its whip on a matter of war and peace - it is now the responsibility of the Scottish National Party to take the lead in holding the UK government to account," the SNP's Westminster leader, Angus Robertson, said. "Given the seriousness of the issue, it is also only right that the prime minister allows for two days of full debate."
Cameron and Merkel's governments look set to beef up their Syria roles almost simultaneously, if to differing degrees
Perhaps Cameron's most bitter parliamentary defeat as prime minister came in 2013, when he sought the House's approval for military action in Syria. Back then, President Bashar al-Assad's military was the desired target. Labour voted almost in lockstep against the measure, while a surprising contingent of 30 Conservative MPs (plus another nine from then-coalition partners the Liberal Democrats) went against the government line.
Prominent Tory MP David Davis, a former serviceman with the territorial division of the SAS special forces, was among the dissenters that day, and seems set to vote against strikes on IS on Wednesday, too.
"Any fight you take on has got to have an end. And you've got to know where it's going to end. And actually we know less about the end of this one than we did about Libya - or than we believed we did about Libya - or indeed the Iraq war," Davis told the BBC's Today program.
Davis said that either Corbyn's decision not to enforce a whip, or Labour's current disunity ("however you want to put it") was what had given Cameron a guaranteed majority, but he stopped short of criticizing the opposition leader: "He is 'new politics' in at least one sense," Davis said of Corbyn. "He's giving everybody the right to say it - and frankly, this is a matter of life and death; you can't whip an issue like this."