veteran soldier; long-serving soldier
Refers to a soldier with long service and experience, often contrasted with new recruits (新兵).
See also: 新兵
The veteran soldier taught the new recruits the mental attitude needed on the battlefield.
退役軍人 is a more general term for a military veteran who has left service, while 古参兵 emphasizes long service within the military.
Compound of 古参 (long service, seniority) and 兵 (soldier).