Old, experienced soldier in US slang
In British slang, an "old sweat" is an old, experienced soldier. What is the American equivalent? I don't mean a veteran (someone who's left the military), I mean someone still in service.
The term "war horse" or "old war horse" is often used to refer to an aged, experienced soldier.
- (Informal) a veteran, as a soldier or politician, of many struggles and conflicts.
(Dictionary.refeence.com)
Soldiers who yearn for the civilian life refer to such as "lifers."