Drafted into the U.S. Army, he was ordered in 1944 to the Graphics Division of the Office of Strategic Services in Washington, D.C., where he made war posters, including "Save Waste Fats" and "Someone Talked", the latter winning an award from the Museum of Modern Art. Shipped to London, he documented, in pen and ink sketches and photographs, everyday life during wartime. After VE Day (8 May 1945), Koerner was reassigned to Germany, working in Wiesbaden and Berlin, and sketching defendants at the Nuremberg trials.