Consider that many developed nations have compulsory military service (Finland, Norway, South Korea, Singapore, Israel, Switzerland, etc.), and yet people still manage to start new companies there.

If the service is obligatory for all, it means you're taking a million of people at the peak of their intellectual capabilities, and force them to do mundane tasks for half a year, or a year. Also, some people will fake their medical records to get out of this.

If it's not obligatory for students (like it was in Poland), people will find some bullshit universities to skip the service. And the vision of being force to essentially be imprisoned for a year if you get thrown out of university, will not be fun for anyone. This is how it looked like in Poland.

But - if I'm not mistaken - what made Poland change the law were those factors: - it's costly. you need to mantain 1% of population fed, clothed and so on for a year or so. plus you need to maintain a huge infrastructure for this. that alone would cost U.S. how much - $15k * 4M = 60 billion? a year? And that doesn't include the fact that those same people would otherwise bring in much more profit to the society - say $40k/year on average. So the hidden cost here goes another 160 billion of a hidden cost in case of U.S. - it's inefficient. the wars are not won by who has more manpower anymore. they are won by the ones who have better technology, intelligence and logistics. So that 4 million army would be blown away by 100 thousand army of better trained and better equipped soldiers.

Finally - look at the countries you mentioned. They aren't exactly known for their entrepreneurial spirit, are they?

If you reduce national service age and compulsory education age to 16, then national service would take place between ages 16 and 20. Beginning university at 20 after four years of life experience would probably be more beneficial than detrimental, and you've subsidized the training of lots of skilled workers in the process.