As batiste suggested below, some countries do have a mandatory army service. As someone who served a bit more than a year in just such an army, let me point out

As batiste suggested below, some countries do have a mandatory army service. As someone who served a bit more than a year in just such an army, let me point out how paying taxes in cash is vastly superior a solution to paying in work: I am an application developer with a very narrow set of "deep" skills. Working as such, I get to charge rates that reflect those skills' depth. If I pay my taxes, I am taxed for some 260 days of work at what I'll call my "peak" rate. If I were to be drafted to do government work, the probability of me working in that very area is extremely small. Even if, every know and then, I'll get that lucky batch of work, I'd still spend a lot of time getting to know the respective code base, the objectives of that project and, after a week or two of acutal work, passing things on. Much more likely, I'd end up in an office somewhere. I am somewhat able to hackishly use Excel, so that might be what I'd do: I'd hack up a couple of unmaintainable spreadsheets to solve some problems which I wouldn't actually be proficient in. Were I to do that work for a client, I might, at best, charge half of aforementioned "peak rate". If that government work were to be sloppily organized, I might even end up working outside, cleaning up a forest or assisting in building a road. As you may imagine, I'd be a terrible fit: While I consider myself healthy, I'm not used to outdoor work. At best, I might accomplish 2 hours' worth of skilled labour in an 8 hour day, at worst I'd screw more shit up than I'd fix. Paying cash, I get to contribute at peak_rate * tax_level and leave the government work to people actually hired for their skills, not the amount of available draft days. Also, I assume many a startup wouldn't take too kindly to missing each of their employees for another month a year. Some countries already do (e.g. Switzerland: 300 days of mandatory military service, done (as in "doing time", rather than "serving") in one part of 21 weeks and some 6 annual parts of three weeks), and while co-workers missing another three weeks per year is certainly bearable, it still seems a mostly useless burden.