The Montreux Convention Regarding the Regime of the Straits is a 1936 agreement that gives Turkey control over the Bosporus and Dardanelles straits and regulates the transit of naval warships. The Convention guarantees the free passage of civilian vessels in peacetime, and restricts the passage of naval ships not belonging to Black Sea states. The terms of the Convention have been a source of controversy over the years, most notably about the Soviet Union's military access to the Mediterranean Sea.Doge
Per CNN, the U.S. is considering sending warships into the Black Sea as a show of force for #Ukraine amidst an increased Russian military presence along the border with #Ukraine. U.S. is required to give 14 days notice of intention to enter, unknown if that's been done yet.
The U.S. is currently discussing the Russian military buildup with NATO allies and have detected more Russian troops on the border with Ukraine than at any time since 2014. Intelligence flights have increased to keep track of naval and ground movements.
Signed on 20 July 1936 at the Montreux Palace in Switzerland, the Convention permitted Turkey to remilitarise the Straits. It went into effect on 9 November 1936 and was registered in the League of Nations Treaty Series on 11 December 1936. It remains in force.
Montreux Convention Regarding the Regime of the Straits - Wikipedia