MPs might not be expert but they are accountable to the rest of us, unlike the Labour Membership.
Since war is still a parliamentary perogative, it would be ultimately MPs voting for it, with the same level of accountabiity to the general public.
Why not just generalize it and ask the membership for approval before making any decision in parliament?
in general the party membership is supposed to set demands, i think it's roughly how the party changed it's brexit policy - the policy for 2nd ref was made by leadership after the members' resolution to back remain and free movement.
@Sweet Square has a lot to say about the democracy (and lack of it) within the labour party.
about whether every parliamentary vote should go back to the party - i haven't thought about it, i would guess that the guideline from the pary conference might be enough for most issues (like, say, welfare spending).
if i had to put this differently, it would be that:
1. the membership passes resolutions the leadership must respond to. the party whip line on various issues is made to correspond roughly with the resolutions.
2. a war (of choice) is an extradordinary event where a general pre-existing resolution may not be enough.
but this is not a solid reason and i'm not wedded to any position really.