The problem with the idea of attacking Turkey is that it doesn’t really gain anything from it. Turkey provides Chromium to Germany through trade and is situated near the Middle East, the Black Sea, and the Caucasus. So their are three options for Germany: Invade Turkey just trade with Turkey, or convince Turkey to enter the war on their side.

Now invading Turkey is dangerous for the Nazi war effort. Not only would this cut off Chrome shipments from them, but this would open another front for the Allies to fight on. This would take German and other Axis troops from the Eastern Front or other fronts to use for the invasion of Turkey. Now Turkey being invaded could ask the Western Allies and Russia for support who are situated in the Middle East and Caucasus. They could easily reinforce the Turks and cause the Germans to retreat and might open another front in the Balkans where Romania and its oil fields could be threatened. It makes no military sense to invade Turkey when the Germans are already busy invading Russia.

Turkey could also be convinced to join the war but once again the Allies are nearby and could easily squash Turkey. This would also require German soldiers to be redirected and help the Turks. Turkey is in no shape to fight a war against the Allies and would avoid fighting in the first place.

The last option of just trading with Germany as a neutral power is the one they went with. Turkey has no long term strategic goals for Germany aside from threatening the Middle East, the oil rich city of Baku and maybe India. But Germany was already spread thin and really couldn’t afford to open up this front. Germany was probably getting ready to invade the ports near the Caucasus on the Black Sea as German units were sent to secure them during the Case Blau offensive. That’s why they had those landing boats in the Caucasus as there is no reason to invade Turkey