Turkey promise to Germany because Soviet Russia danger for eastern cities of Turkey. Germany wait for invade to Turkey. Because they are thinking ‘may be Turkey enter the war with us’. Meanwhile Germany invade to Russia and Russia win. And Germany didn't invade Turkey. End of the history. This is Academic. read more
Invade Thrace (European Turkey, west of the Bosphorus) to set up airbases and control the Bosphorus Strait. Blockade and bombard the coastal cities and towns. This would require sea power, which Germany didn't have in this part of the world. read more
the tldr here is that Germany didn't invade Turkey because it could not do so in any practical sense. edit: One other thing. This post used the Greek invasion as a convenient metaphor for what a Turkish invasion would have looked like. But Turkey was not Greece. read more