A recent letter in my newspaper posed a stark warning - if the West runs down stocks of 'conventional' weapons by supporting Ukraine, maybe nuclear arms will be all that's left.
I sympathise, especially as Russia is likely to be dangerously unstable is Putin's regime collapses. However, defence considerations should not be confused with David Lammy's suggestion that compulsory national service would help break down divisions in society. Historically, military conscription has tended to be a response to external threats which tend to unify society in and of themselves, as in Scandanavia (or Britain in 1939). Despite the idea of conscription as unifier dating back at least to the French Revolution it has rarely been attempted for that purpose, making it hard to judge Lammy's proposal by reference to past examples. The case of Israel is interesting, because unity against outside threats - alongside conscription - has not prevented intensifying political and social divisions.
I suggest that if the aim is frankly social rather than defence, it would probably be safer to introduce a broad community service, with military service as an option. 