Labour are at war over Syria: how much damage is inflicted on them is at least partly down to where they land on the subject. But from where they are, I can’t see any possible good outcome. The best one from their perspective would be a collective vote for the strikes on the proviso that certain key elements are kept to, most notably what comes next and how ISIS actually ends up being defeated. But this does not look possible given Corbyn and McDonnell’s unbending opposition to such a position. So you’re left with four other possibilities: a free vote on the matter; Corbyn whips a vote against the strikes and then the whip is followed begrudgingly; Corbyn whips the vote and there are mass rebellions, which would involve a lot of shadow cabinet resignations; Corbyn is forced out of the leadership on the issue.
We can discard the last one almost out of hand – Corbyn is going nowhere in a hurry. Bear in mind, he thinks of the PLP as an annoying distraction at the best of times, so whatever they do or don’t do is almost meaningless to him in the larger picture. So let’s look at the other three possibilities.
Corbyn whipping and everyone following is not out of the question. Hilary Benn has already alluded to this possibility with his “I disagree with everything Jeremy says or thinks but I am behind him 100%” rhetoric. It could happen. The upshot of this is that the left of the party scores a huge victory and Labour looks even weaker on national defence (somewhat unfairly in some respects, but it would stick nonetheless). This is weirdly possibly the best outcome, but it’s still a pretty bad one for Labour.
The other two involve total mayhem. If there is a whip and then it is rebelled against en mass, you will see the total meltdown of the Labour Party, at least in the short term. Who does Corbyn get in to take up the shadow cabinet posts suddenly vacant? How can Corbyn get round the fact that his parliamentary party are visibly not behind him?
The final one is the free vote. Which looks like the easy solution, which why Benn amongst others are pushing for it. But it is actually a very bad option for Labour. They’ll look divided – they will by definition when half the party votes with the government. They’ll also look weak and indecisive on a very important issue. Furthermore, it sets a bad precedent. Will Labour start having free votes on everything? If they can have one on something as vital as whether or not we bomb someone, why not have one on everything else? Then, the PLP is effectively meaningless; they may as well all be left-wing independents if the party as a whole never takes a collective position on anything.
Also a free vote would just be kicking the can down the road – the yawning chasm between the PLP and its leader cannot continue indefinitely without massive long term consequences.