As the outbreak of World War II approached, Nazi Germany ordered artillery manufacturers Krupp and Rheimetall-Borsig to build several super-heavy siege guns, vital to smash through French and Belgian fortresses that stood in the way of the Blitzkrieg. These 'secret weapons' were much larger than the siege artillery of World War I and included the largest artillery piece of the war, the massive 80cm railway gun 'schwere Gustav' (Heavy Gustav). However, these complex and massive artillery pieces required years to build and test and, as war drew near, the German High Command hastily brought several WWI-era heavy artillery pieces back into service and then purchased, and later confiscated, a large number of Czech Skoda mortars.