I know it's already been specified in the OP that the situation involves an invasion not a war of aggression, but there was something I read once about drafts that I would love to introduce in the UK.
If there is to be a war and it involves a hostile invader then a straight up draft but if it involves a foreign war, then have a public vote. Only those eligable for drafting gets to vote. If the votes say yes, then go to war and draft everyone who voted yes immediately. If more soldiers are needed, draft the ones who were eligable but didn't vote. Finally draft the ones who voted no if more ae needed.
I can't remember where I read about this system but it sounds like a system that, if a draft were ever to be instated, would be both fair and ensure politicians cannot abuse it easily.
If there is to be a war and it involves a hostile invader then a straight up draft but if it involves a foreign war, then have a public vote. Only those eligable for drafting gets to vote. If the votes say yes, then go to war and draft everyone who voted yes immediately. If more soldiers are needed, draft the ones who were eligable but didn't vote. Finally draft the ones who voted no if more ae needed.
I can't remember where I read about this system but it sounds like a system that, if a draft were ever to be instated, would be both fair and ensure politicians cannot abuse it easily.