I accept that there's a case to answer for the PM and cabinet taking this action without parliamentary approval, but as I understand it, it is not actually required.
Here are legitimate reasons for not seeking parliament's consent:
- a debate in parliament with a 'yes' vote telegraphs to the Syrians that the bombs are coming and puts
UK service personnel lives at risk, as well as potentially limiting the effectiveness of the strike - for operational reasons the PM was never going to tell parliament exactly what was going to happen
anyway
Here's a less legitimate reason:
- She knew she might not get the necessary approval
I suspect a bit of all three, but I accept that she didn't actually need parliament's approval either. The office of Prime Minister entitles her to take the responsibility for these decisions herself.
I can accept a bit of disquiet about it from the moderate left, but I find the hard left's objections extremely hypocritical and distasteful. It seems to me that Corbyn, Abbott, Provo McDonnell and the rest will always view any military action through a leftard lens in which the West is necessarily the brutal bad guy, helping itself to a bit of imperialistic aggression.
Compare and contrast Corbyn's reaction to the government for carrying out these strikes, in which as far as we know no-one was even injured, to his reaction to a terrorist group deploying a large bomb aimed at murdering the British cabinet, in 1984; an action which killed five people, and which was carried out without the democratic consent or approval of anyone except themselves, a criminal gang.
No hand-wringing anger on that occasion, oh no. Instead he invites a few of them round to the Commons, and he's been on friendly, supportive terms with them ever since.

I'm afraid the blunt truth is that Corbyn and friends would never support any form of armed action taken by their own country. Only against it.