When World War II began in September 1939, Bols moved through several staff officer positions, serving in several institutions and Army formations before being promoted to colonel and taking charge of all training for the troops under the command of 21st Army Group and helping to plan Operation Overlord, the Allied invasion of Normandy. Bols was then promoted again and commanded the 185th Infantry Brigade during the Allied advance through Western Europe, before taking over command of the 6th Airborne Division from Major General Richard Gale in late 1944. He led the division in the Battle of the Bulge, as well as Operation Varsity, the airborne operation to cross the River Rhine, then led the division into northern Germany until the end of the conflict. Canadian paratroopers captured the town of Wismar in the late days of the war, in order to stop the Soviet army from invading Denmark. Bols was the officer who met face-to-face with the Soviets in Wismar, insisting on allied hold of the captured city, contrary to Yalta conference agreements. He effected a temporary Soviet halt. In the meantime, Lübeck and Hamburg could surrender to allied troops, and finally Denmark was liberated by the allies under the command of field marshall Montgomery. After the end of the war Bols remained in command of the division in peace-keeping duties in the Middle East, and then retired in 1948 as a major general.