The Sea of Azov, east and northeast of the Crimean Peninsula, is the shallowest sea on the earth. Please note the narrow water entrance between Kerch on the Crimean side and what looks like a turtle head about ready to snap up Kerch across the narrow water channel.
Now note the narrow water entrance of the Black Sea seen at Istanbul between Turkey and the European Mainland.
The Cold War Soviet Union conducted well-coordinated land, sea, and air operations with rival friendly Soviet divisions on opposite sides of a water channel between Kerch and the turtle head. They did this to be able to quickly conquer the Istanbul channel if war broke out. Turkey was a trustworthy ally of NATO during the Cold War.