When referring to U.S. Forces, the term "soldier" is not generic. The term soldier refers specifically to someone serving in the Army.
Here's a primer for you:
U.S. Army = soldier
U.S. Air Force = airman
U.S. Navy = sailor or seaman (snicker)
U.S. Marine Corps = Marine
According to AP Style and all-but recent convention, the only one of these nouns that is proper is "Marine." However, it has become common among the services to capitalize Soldier, Airman and Sailor also.
Generic terms for all services include: service member, military member, troop (which is always refers to more than one person, and is specifically used to define a company-sized element in U.S. Army Cavalry units).
Please listen. Current and former military folks alike laugh at you when you make the mistake.