The strategic importance of the Bosporus remains high, and control over it has been an objective of a number of hostilities in modern history, notably the Russo–Turkish War, 1877–1878, as well as of the attack of the Allied Powers on the Dardanelles in 1915 in the course of the First World War.
Several international treaties have governed vessels using the waters. Following WW I, the 1920 Treaty of Sèvres demilitarized the strait and made it an international territory under the control of the League of Nations. This was amended under the 1923 Treaty of Lausanne, which restored the straits to Turkish territory – but allowed all foreign warships and commercial shipping to traverse the straits freely. Turkey eventually rejected the terms of that treaty, and subsequently Turkey remilitarized the straits area. The reversion to this old regime was formalized under the Montreux Convention Regarding the Regime of the Turkish Straits of July 1936. That convention, which is still in practical force as of 2008, treats the straits as an international shipping lane, but Turkey does retain the right to restrict the naval traffic of non-Black Sea nations (such as Greece, a traditional enemy, or Algeria).
Ah, it was another convention signed at roughly the same time. Thanks for the correction.