Granted, Wikipedia is not always a reliable source, but I also checked this in other on-line and off-line dictionaries. Soldiers are most often associated with an army, especially in the US of A.
Xzins, I am a soldier, as are you.
Xzins, I know you are our fine Chappy, and I honor you as such.
However, a Marine is not a soldier, a Marine is a Marine.
Need I say more, Chappy?
I was in the US Army and the US Marine Corps so I was a Soldier, but I am still a Marine. Confusing ain’t it?