Mandatory military service
In the States, there’s no compulsion to enlist in the military. You do so out of your love for the country or for a lack of anything else to do. Serving in the army for a set number of years can also help you pay for school after being discharged. In South Korea, there’s no such thing as voluntary military service.
Every able-bodied man between the ages of 18 and 28 has to serve in the military for at least 21 months, no ifs, ands, or buts. You can defer your service if you have a legitimate reason (being an actor, professional gamer, health-related reasons), but you absolutely must enlist before turning 28. There’s no forced military service for foreigners, so travelers are completely safe.