German offensive ability had already been crippled after the failure of Operation Barbarossa and the Battle of Moscow in December 1941. From then on, choises had to made where to strike. This resulted into Case Blue in 1942, when an economical objective was preferred over a political or military-strategical. Trying to gain time by slowing down the growth of Soviet war economy, by cutting off the oil supply from the oil fields near the Caspian Sea region to the factories in the Urals (or, preferrably, seize these oil fields) was at least a questionable choice, given the complexity of the operation. Then, there was a need to establish a defence line to cover the northern flank of the offensive, and this defence line hooked on the Wolga near a city, incidentally called Stalingrad. It was neither planned nor needed in Case Blue to take the city, but its namesake was too tempting for Hitler, so he drew more and more troops from the primary objective, in order to take part in the totally useless siege of Stalingrad, jeopardising the real objective of the offensive, which ultimately failed too.It pretty much ended German offensive ability in the East.