In 1945, Calero's company engaged in combat against a squad of German soldiers in what is known as the Battle of Colmar Pocket in the vicinity of Colmar, France. Calero attacked the enemy squad, killing 10 and capturing 21 enemy soldiers before being wounded. For these actions, he was awarded the Silver Star Medal and nicknamed "One-Man Army" by his comrades. Calero was wounded four times during combat in Europe. He was awarded 22 decorations and medals for his actions, making him one of the most decorated Hispanic soldiers in the U.S. military during World War II. Among his many decorations were the Silver Star Medal, four Purple Hearts and the French Croix de guerre.