Not only South Korea has mandatory military service, but several other countries do as well, including Thailand. The difference is that in South Korea, all men are required, while in Thailand they must first go through a selection process. The method is through a lottery or some sort of drawing. Every year, men who have reached the appropriate age must gather at their respective selection locations. Those who receive a red card are obligated to join the military, while those who receive a black card are exempt.
Interestingly, during each selection, some transgender individuals are seen participating and attracting attention. Here are some photos of them!