In the WWII armistice talks, Japan agreed to disband any standing armies, and now only have a small defensive force, if I recall my history correctly.
It is a demilitarised nation.They have a very small national defense. I think all males must serve a certain amount of time.
Japan does not have mandatory military service. Maybe you confused it with Korea?
They have a defensive force and their military forces don't fire weapons outside japan. So all the military forces deployed overseas are non-combatant. By Japanese law they can't declare war or use military force in international disputes. But as of 2015, Japan's parliament voted into law a defence policy shift which could led to their troops to combat overseas.
It's a shame since it goes against the pacifist constitution, created to prevent that the events of WWII happen again.
I think that this is the first manga I have read where someone is in the military during modern times in Japan. For some reason I never thought about it, even though I am normally very curious about other nations military.