Born on 20 August 1893, Rackow was introduced into the Cadet Corps at an early point of his life. He joined the Imperial German Army's 158th Paderborn Infantry Regiment as "Fahnenjunker" (officer cadet) in 1912. By the time World War I broke out in 1914, he had risen to second lieutenant. Rackow took part in the initial German invasion of Belgium and France. He served with distinction as adjutant, fighting at Charleroi, St. Quentin, and taking part in the First Battle of the Marne, First Battle of the Aisne, and the First Battle of Artois. In September 1915, he was appointed commander of the 158th Paderborn Infantry Regiment's 3rd company. With this unit, he participated in the Second Battle of Champagne during which he was wounded twice. In the first case, he was hit by shrapnel in his right foot, but fought on, only to be heavily wounded north of Le Mesnil three days later. He was hospitalized, and returned to the 3rd company in February 1916.