Why did Fall Blau start at Voronezh instead of further south?

Kudos to Bobby House on Quora for providing "the" answer to "Why Was Stalingrad So Hard to Capture In World War II?" My answer builds on his, which I cannot fully link to on Quora.

Voronezh was an essential element of an earlier, more limited version of Fall Blau, contained in Hitler's Directive 41. The economic purpose of the original plan was to cut off the Soviets from oil and other supplies, and only secondarily, to obtain them for Germany.

To this end, the first step was to establish a northern anchor of the German southern front, on the Don River. Voronzezh was a city just east of the Don, a good place for just this anchor. Because Stalin had been deceived into thinking that the main German thrust would be toward Moscow, the defeated Russian troops would either be pushed north, out of the way of the German offensive, or "trapped" if they tried to move south in front of the main German thrust discussed below.

The second step was to use some of the victorious Germans of Army Group B (Hoth's Fourth Panzer Army) to advance east from the Don to the Volga, and capture or isolate Stalingrad on its way south (and east). Later, it was Hitler who contravened his earlier order, and directed Hoth to bypass Stalingrad on his way south.

With Stalingrad in the "bag," Army Group A to the south would surge forward (east) and join Hoth's group in capturing the west bank of the Volga between Stalingrad and Astrakhan. This would interdict Russian oil shipments going north along the Caspian sea, and up the Volga, or along railroads on the east bank of the Volga, parallel to the river.

Only after these goals had been achieved, and Army Group B was firmly ensconced along the lower Volga and middle to lower Don, would Army Group A "split off" south into the Caucasus, hopefully capturing Maikop (which they did) and Grozny (which they nearly did).

The timetable would have ruled out an advance on Baku, at least in 1942, but Germany did not have a realistic hope of capturing it anyway. The oilfields at Maikop and Grozny could have been restored in late 1943, early to enough to help Germany, if it had succeeded in holding its positions on the Don and Volga.

During the course of the plan, Hitler became more concerned about capturing oil for Germany than denying it to the Russians, so he skipped the "intermediate" phases of Fall Blau (the Stalingrad to Astrakhan part), and ordered Hoth to go due south to help Army Group A break through at Rostov. As it were, the two German armies got in each other's way around Rostov, while the Russians were able to reinforce Stalingrad "and the rest is history."

The "fly in the ointment," is that the Russian generals (after the disastrous battle of Kharkov), did not leave large numbers of troops to be surrounded and captured; the Soviet army would live to fight another day. Under this version of Fall Blau, the 1942 campaign might have been "trivial;" that is, the Germans would have taken and held all the land they actually took, plus Grozny, and a stretch of the lower Volga between Stalingrad and Astrakhan. The "battle of Stalingrad" might have been a replay of Voronezh, a "small" battle, not a turning point in the war.

Such success in the original Fall Blau would not have won the war for Germany. But it would have prolonged it for at least a year (absent the atomic bomb), because it would have taken the Soviets at least until mid-1944 to get back to their actual "start" line around Kursk of the summer of 1943. Also, with more oil from Maikop and Grozny, the Germans might not have succumbed to the offensives of the Western Allies as quickly as they did. Meanwhile, the Russians would still have their oil at Baku, but would have to ship it via a "long" route, east through Iran, then through Kazakhstan, and then back to Russia. Ditto for Allied "Lend Lease" supplies arriving in Iran.

Destruction of the Red Army and a feint towards Moscow

While it is absolutely true that oil was the primary strategic goal of Fall Blau and indeed of whole the German offensive effort in 1942, way to achieve that goal was more complex. The Germans did enjoy a slight numerical superiority at the beginning of Barbarossa, due to the Soviet losses and piecemeal deployment of raw replacements, they held that advantage until roughly December of 1941. It was no longer the case in 1942, so they had to select a section of the front where to attack, concentrate forces there without giving the Soviets advance notice, encircle and destroy a large part of the Red Army, and somehow prevent a subsequent large counter-offensive.

Now, just by looking at the map, it is clear that if you intend to drive towards Maykop, Grozny and Baku, distances would be enormous and already long front lines would be even longer. If you leave Soviet troops near Voronezh (which were initially part of Bryansk Front) unmolested, they could strike at your flank at an opportune time and cut you off. It is much better to destroy them or push them to the left bank of the Don, and use river as a natural obstacle to cover your advancing forces.

One more thing to consider is deception: The Soviets believed, and Germans did everything possible to reinforce this belief, that the major German effort in 1942 would be towards Moscow. To this end, Germans even organized Fall Kreml, a large deceptive effort to persuade the Soviets to keep large formations at Moscow direction. Even as Fall Blau unfolded, there was still opportunity to go to north-northeast direction towards Moscow from Voronezh. This illusion was further reinforced when during the battle of Voronezh German forces briefly crossed the river to the left bank of the Don.

One final thing to note - the Germans were hoping to repeat their large encirclement battles of the previous summer. When this did not materialize on the scale they wished, they started deluding themselves that the Soviets were near the end of their manpower reserves. German leadership knew that if the Red Army was not destroyed, a Soviet winter counter-offensive was bound to happen somewhere (and historically major efforts were at two places - Stalingrad and Rzhev). Therefore, before trying to capture any oil, it was prudent to shatter as much as possible of the Red Army. In the first part of the campaign, German efforts were aimed more at this goal; and then somewhere from mid July, they started to move towards their primary objectives.