(someone) is required to (do something)

If you "are required" to do something, that means that you have to do it:

All members are required to attend.

Borrowers are required to provide proof of their income.

This is a formal phrase. A more casual version is:

All Korean men have to serve in the army.

serve in (the military)

The job that members of the military do is called "service". We say that they "are serving their country" because "service" is a kind of work that you do in order to help other people in your society:

It's important that we honor those who serve their country.

To specify which part of the military a person works in, use the phrase "serve in ___":

I served in the Self Defense Force for twelve years.

We also use the word "serve" to describe other jobs that help society, but some use "on" instead of "in":

He served on the New York Police force for over 30 years.

the army

There are different types of military forces. In the United States, we have:

• the Army, which fights battles on the ground.
• the Navy, which operates in the ocean
• the Marines, who are trained to fight between the ocean and the land.
• the Air Force, which does attacks from the air.

But when people are speaking in general about all of these groups, they usually call them "the military" (which is the correct term) or "the army" (which is incorrect but often used as a general term for all military branches).