For much of the 20th century, photography was the single most powerful method for conveying the horrors, triumphs, epic challenges, and small, daily struggles of warfare. In searing, unforgettable pictures, great photojournalists quite literally brought war — every war — home to millions of Americans. Here, LIFE.com presents 50 extraordinary photographs from three 20th-century conflicts that, in many respects, were defined by the way LIFE magazine covered them: images from World War II, Korea, and Vietnam that, collectively, serve as a testament to those who fought, and an admonition to those seeing the pictures today: Remember. ABOVE: Marine Gunnery Sgt. Jeremiah Purdie (left), wounded in a firefight during “Operation Prairie” in Vietnam, reaches out to a stricken comrade in Larry Burrows’ astonishing 1966 photograph. Here, in what might be the greatest picture from a legendary career, Burrows captured for LIFE magazine’s millions of readers both unfathomable desolation and galvanic camaraderie in the utterly alien universe of Southeast Asia. That the image, made at the height of the Civil Rights era, depicts a black soldier desperately trying to aid a wounded White comrade only added resonance to an already emotionally devastating tableau.