One was diverting Army group Center’s panzer armies to the other army groups in July-August. Army group center had just taken Smolensk and was in striking distance of Moscow. However Hitler diverted its two panzer armies to the South and North to destroy the soviets in Kiev and the Baltics.

The historical debate about Moscow vs Kiev is misleading: The Wehrmacht was not in a position to mount an early attack on Moscow. In September 1941, Army Group Center was still in the process of building up a supply stockpile that would enable them to attack Moscow. Until this stockpile was complete, it was logistically impossible for them to launch an offensive on Moscow. Under those circumstances, it was only logical that they go after the closer and easier objective of Kiev.

And BTW, diverting elements from Army Group Center was inherent to the plan for Operation Barbarossa. They were always supposed to clear their flanks before advancing on Moscow. Hitler was within his rights when he issued Fuhrer Directive 33. Halder attempted to change Hitlers mind, but he remained adament.

Hitler then foolishly split the army group into two weaker groups: Army group B which would take Stalingrad and form a line with the Volga and Army Group A was to penetrate the caucasus and capture Baku. These diluted forces failed to achieve either objective and the offensive bogged down.

In this instance also, the splitting of Army Group South into A and B was inherent to the plan for Case Blue. Originally, Army Group B was supposed to advance along the Don river and capture Stalingrad, securing their flanks against the Soviets: Only then would Army Group A mount its advance into the Caucasus. The problem is that Hitler threw the entire operation out of sequence when he issued Fuhrer Directive 45. He ordered the advance into the Caucasus to begin before Army Group B was anywhere near Stalingrad!

Von Bock tried to get Hitler to rescind the order, but he got fired instead. Army Group B could have taken Stalingrad in August 1942, but instead, they didn't even reach the city until September 1942, and by then, the Soviets had turned the city into a fortress. This meant that the Wehrmachst flanks were never secured, which opened them up to a catastrophic defeat.