A friendly local talking to German soldiers (Bundesarchiv)
Fall Blau (Case Blue) was the German codename used by the German Armed Forces (Wehrmacht) for its 1942 strategic summer offensive in southern Russia that lasted between 28 June and 19 August 1942. The offensive was so named because German military plans were "cases", or solutions to problems. The operation was a continuation of Unternehmen Barbarossa (Operation Barbarossa).
The German offensive faced two problems: the continued resistance of the Red Army which now occupied a defensive position west of the Volga river, and the demand by Adolf Hitler for securing the Caucasus oil fields, which had to be reached by crossing the Caucasus mountains.
Initially the German offensive met with spectacular gains. However, the Red Army defeated the German Army at Stalingrad. This defeat forced the Germans to retreat from the Caucasus for fear of being trapped themselves
Crossing a river (Bundesarchiv)