We have to refer to the Montreux Convention, which regulates naval shipping on the Bosporus and allows transit through the seaway. This allows NATO ships to travel to the Black Sea and Russian ships—at that time, they were Soviet ships—to pass and use the Bosporus seaway.
It's an important strategic route. I'm not going to give you a history lesson, but we can refer to the Battle of the Dardanelles. It was already strategically important.
As you know, part of the NATO fleet was active in the Black Sea, and it still is. It must pass through the Bosporus, which is controlled under the Montreux Convention.