Back in 2011, it was promised by the current Foreign Secretary that the Commons would be given a vote before any military action and the Cabinet manual was changed to reflect this (with the caveat “except when there was an emergency and such action would not be appropriate”). It is now accepted to be a convention by the government, and more than that they proposed to enforce it by statute.
Now it appears that civil servants are struggling to come up with a bill that would give ministers the flexibility they require along with ensuring ultimate authority is with MPs. Former generals appear to be leading the charge against, with the message from the article being that if the bill is delayed beyond this Spring, it is not certain to become law thanks to tenacious former generals in the Lords who appear to also be lobbying against even an unenforceable convention.
The House of Commons has only recently accrued any kind of power over armed deployments: before 2003’s invasion of Iraq, only the Korean war in 1950 had received a vote of the House of Commons. Since then another intervention, this time in Libya, has received a vote of the Commons. Last Parliament, the Lords’ Constitution Committee published an exhaustive report on the issue of war powers for the Commons. It’s well worth a read whatever side of the argument you lie on, but essentially it proposed a convention should be formed:
(1) Government should seek Parliamentary approval (for example, in the House of Commons, by the laying of a resolution) if it is proposing the deployment of British forces outside the United Kingdom into actual or potential armed conflict;
(2) In seeking approval, the Government should indicate the deployment’s objectives, its legal basis, likely duration and, in general terms, an estimate of its size;
(3) If, for reasons of emergency and security, such prior application is impossible, the Government should provide retrospective information within 7 days of its commencement or as soon as it is feasible, at which point the process in (1) should be followed;
(4) The Government, as a matter of course, should keep Parliament informed of the progress of such deployments and, if their nature or objectives alter significantly should seek a renewal of the approval.
Whether there will be a bill to enforce this is now looking uncertain.