In the spring of 1942, Hitler placed major emphasis in the summer campaign on the southern section of the German-Soviet Front in Operation Blau. Hitler sent General Fedo von Bock's Army Group South east to secure Vorovezh first (fell on July 6), and then reorganized the southern forces into Army Groups A and B. General Siegmund W. List commanded southern formation, Army Group A, while General Maximillian von Werchs commanded northern formation, Group B. He originally planned for Groups A and B to cooperate in a large effort to secure the Don and Donets Valleys and capture the cities of Rostov and Stalingrad. They would then capture the oil fields in the south which were vital to the Red Army. On July 13, Hitler changed the plan and ordered both armies to seize Stalingrad and the Caucasus. This actually enabled Soviet troops caught in the Don River bend to escape eastward, because of the gap in the Army Groups. On August 23 the Sixth Army reached the Volga River north of Stalingrad. Their original intent was to control the river by gunfire and destroy the arms factories, notably the Tractor, Red October, and Barricade works.