During World War II, Voronezh was the scene of fierce fighting between Russian and combined Axis troops. The Germans used it as a staging area for their attack on Stalingrad, and made it a key crossing point on the Don River. In June 1941, two BM-13 (Fighting machine #13 "Katyusha") artillery installations were built at the Voronezh excavator factory. In July, the construction of "Katyushas" was rationalized so that their manufacture became easier and the time of volley repetition was shortened from five minutes to fifteen seconds. More than 300 BM-13 units manufactured in Voronezh were used in a counterattack near Moscow in December 1941. In October 22, 1941, the advance of the German troops prompted the establishment of a defense committee in the city. On November 7, 1941, there was a troop parade, devoted to the anniversary of the October Revolution. Only three such parades were organized that year: in Moscow, Kuybyshev, and Voronezh. In late June 1942, the city was attacked by German and Hungarian forces. In response, Soviet forces formed the Voronezh Front. By July 6, the German army occupied the western river-bank suburbs before being subjected to a fierce Soviet counter-attack. By July 24 the frontline had stabilised along the Voronezh River as the German forces continued southeast into the Great Bend of the Don. The attack on Voronezh represented the first phase of the German Army's 1942 campaign in the Soviet Union, codenamed Case Blue.