In the late 1950s and early 1960s war comic books became more popular, mirroring trends in pop culture as a whole. As the World War II generation moved into positions of power, they naturally wanted to explore the themes of the most important events of their generation. DC had several war magazines as the Silver Age started; All American Men of War, Our Army at War, Our Fighting Forces, and Star Spangled War Stories; they added GI Combat after purchasing the title from the old Quality Line. For the most part the stories in these comics were one-shots, featuring characters that would not appear again. This was a significant negative as most other comics had continuing characters.
So DC gradually began to add features to its titles which did not have them. Our Fighting Forces starred Gunner & Sarge starting with issue #45, with a May 1959 cover date. Gunner was a Thompson sub-machine gunner, while Sarge was the crusty order giver. The pair apparently formed the smallest squadron in the US Marines, although they grew a bit with Our Fighting Forces #49, when a German shepherd dog (named Billy, but simply called Pooch) was added to the mix. Although they mostly fought in the Pacific, oddly enough in the first story they were in Europe battling the Nazis.
That story also established a basic pattern for the series. Gunner was the decoy, intended to draw the fire of the enemy so that the Sarge could locate and finish them off. Many stories featured Gunner griping about this.
Gunner, Sarge & Pooch lasted until Our Fighting Forces #94, August 1965, when they were retired in favor of the Fighting Devil Dog, a series of Vietnam stories featuring Sergeant Rock's younger brother, Larry. However, they were resurrected a few years later for a series called "The Losers" which featured them, Johnny Cloud, and Captain Storm. Ironically, The Losers lasted even longer than Gunner & Sarge's original fifty issues, accounting for the final 59 issues of Our Fighting Forces.
Update: Star-Studded War Comics has a post up featuring a solid Gunner & Sarge story.