During World War II the regiment fought as part of the 9th Infantry Division.

The Fighting Falcons of the 39th became the first unit of United States combat troops to set foot on foreign soil when they stormed the beaches of Algiers in November 1942.

During fighting in Sicily, Italy, the regiment came under the command of the legendary Colonel Harry A. "Paddy" Flint who gave the regiment its triple A- Bar Nothing slogan …Anything, Anywhere, Anytime - Bar Nothing. The regiment took great pride in the AAA-O slogan, displaying it on their helmets and vehicles, even in combat. When questioned about the soundness of the practice, Colonel Flint confidently declared, "The enemy who sees our regiment in combat, if they live through the battle, will know to run the next time they see us coming."

General George Patton commented upon Colonel Flint in the following manner: "Paddy Flint is clearly nuts, but he fights well." On 31 July 1943, while temporarily attached to the 1st Infantry Division), the 39th suffered its first serious reverse at the battle of Troina, when entrenched and heavily-armed German forces repelled an assault by the 39th Infantry Regiment with heavy casualties.

Later in the war, the 39th landed at Utah Beach and fought through France. The Fighting Falcons joined the 47th Infantry Regiment in capturing Roetgen, the first German town to fall in World War II. The 39th fought valiantly through the Battle of the Bulge, helped secure the Remagen bridgehead and roared across Germany as the allied forces finished off the last of the German resistance.

When the dust settled following VE day, the 39th Regiment held campaign streamers from some of the bloodiest and most hard fought battles of the war - Algeria, Tunisia, Sicily, Normandy, Northern France, The Rhineland, Ardennes-Alsace, and Central Europe. It was cited twice by the Belgians for valorous actions and awarded the Belgian Fourragère. It also received two French Croix de Guerre with Palm, the French Fourragère, and three Presidential Unit Citations. It was while fighting in France that the regiment's commander, Col. Flint, was killed in action.