We had it after WW2.
I think we may need it again, although I'd design it differently.
Instead of it being a specifically military thing, I'd recommend training of a variety of types, all of which might be used to assist in a war if one comes. But also in the event of another pandemic and so on. Some of the skills would also have commercial applications - things like nursing, plumbing or computer engineering. But why not harness skills like computer gaming skills and drone flying skills that will be needed in the event of a war.
Maybe start by training trainers. Then train trainees as they reach 18.
We need an awful lot of people pre-trained and available for challenging situations these days, instead of always being responsive, which is to say slow.
And with the situation in Ukraine, we need to get ahead of the curve. Like countries such as Finland and Denmark.