In general, there are two major arguments in favor of this proposition. You should base your essay on them.
First, there is the idea of civic duty. You should talk about how every citizen has a duty to do something to help society. Getting military training is one of these because it prepares every person to be able to defend the country. This argument would be strongest, of course, in countries that are actually in danger of having to defend themselves.
Second, there is the idea of discipline. You can argue that getting military training improves a person's self-discipline. You learn to do things like following orders and putting your own desires behind the needs of the group. There is also the physical/psychological discipline needed to deal with hard exercise and exertion. These things can be gained from military training.
You can produce a nation of warriors to follow orders without question. <sarcasm emphasis>