An 87-year-old deal is keeping a lid on the naval war in Ukraine — for now

• Fighting between Russia and Ukraine in the Black Sea has picked up in recent months.
• The fighting is taking a toll on Russia's Black Sea Fleet, which Moscow can't reinforce.
• Turkey's enforcement of the Montreux Convention limits what Russia and NATO can send into the sea.

While Ukrainian and Russian forces are locked in a grinding battle for territory in eastern and southern Ukraine, fighting in the Black Sea has picked up.