Common Borders, Uncommon Rivals
Black Sea; Russia has the largest navy, Ukraine has the longest coastline but Turkey controls the shipping and naval access to it. Here's a blast from the past.
Turkey’s announcement of shutting down access to the Black Sea for naval ships, effectively cutting off Russia’s Black Sea Fleet based in Sevastopol, reminds us of the nearly 500 years-long rivalries between Russia, Turkey, and Iran for the domination of the Black Sea and Caspian regions and how Russia and Ottoman Turkey once joined forces to cut Iran off its the Black Sea region.1
In the summer of 1724, Russia and Ottoman Turkey signed a treaty at Constantinople (Istanbul) to divide the Iranian territories between the Black Sea and the Caspian; the whole of the Caucasus region between them.2
In the 1720s, Iran descended into a protracted civil war. Capitalizing on this, Russia’s Peter the Great attacked northern Iran (Russo-Iranian War, 1722-23) and occupied Iranian provinces of Dagestan, Baku, Gilan, Mazandaran, and Astrabad. Not to be left behind; Ottoman Turkey invaded from the west, occupying Georgia, Western Azerbaijan, and Armenia. With this, Iran was completely cut off from the Black Sea and the Caspian.
The following year, Russia and Ottoman Turkey signed the Treaty of Constantinople (1724) to formalize this occupation and force Iran to accept the status quo. They had overlooked one minor detail; a meteorically rising Iranian army commander named Nader Afshar.
PS: just five years later, Nader Shah Afshar would hand Ottoman Turkey its most humiliating battle defeat ever and recover the occupied territories and access to the Black Sea. The Russians, having just witnessed the fate of the Uzbeks, Afghans, and the mighty Ottoman army, would retreat from Iranian Caucasus, Gilan, and Mazandaran without a fight.3
A.V. Boldyrev (2018) Russia, Turkey, and the Problem of the Black Sea Straits in 1898–1908, Russian Studies in History, 57:2, 162-180, DOI: 10.1080/10611983.2018.1586391
Bain, R.N. Slavonic Europe - A Political History of Poland from 1447 to 1796. Read Books, 2006. https://books.google.co.in/books?id=PdZBZcUbSewC.