Robert Card, the guy cops say is the Maine mass shooter, has a military background ... earning medals in the Army Reserve despite never seeing combat.

Card enlisted in the Army Reserve way back in 2002, working as a petroleum supply specialist.

During his service, Card was awarded several medals ... including the Army Achievement Medal, the National Defense Service Medal, the Humanitarian Service Medal, 2 Army Reserve Component Achievement Medals and the Army Service Ribbon.

Card's duties as a petroleum supply specialist made him responsible for receiving, storing and shipping petroleum-based products ... making sure the fuel was transported and handled safely and properly dispensed in various Armed Forces vehicles and aircraft.

The Army Reserve's online job listing for Card's position promotes the job like this ... "As a Petroleum Supply Specialist, you’ll supply the Army with the fuel it needs to maintain a state of readiness at all times."

As we reported ... cops are on the hunt for Card, with police saying he went on a shooting rampage at a bowling alley and a bar in Maine, killing at least 18 people and injuring 13.