You're thinking conscription. E.g. - we go to war and need new people so we just start grabbing any able bodied man, and even woman in some countries, to train up and deploy. Compulsory enlistment is different. What generally happens is that when you reach the age of legal adulthood you are required to enlist for a certain period of time. This is generally 1-2 years. You do your training, serve your time, and are discharged.

During your enlistment you not only trained but also of course compensated and generally very well prepared to lead a productive life. I think there's a lot to admire about this. But yeah - the issue is when these ideals mix with the reality of enlistment turning into deployment in wars of dubious merit, which is why this whole idea is a nonstarter for the US.

But one example of a nation where it does work well is Switzerland. The reason I mention them in particular is because in 2013 there was a referendum aimed at abolishing this enlistment. It failed with more than 73% voting against it. [1] I think it indicates how well it can work and the value it can provide, but of course again the big difference is that Switzerland is also incredibly conservative in the deployment of their military.

[1] - https://en.wikipedia.org/wiki/Conscription_in_Switzerland