Montreux Convention Regarding the Regime of the Turkish Straits

The International Straits Commission was abolished, authorising the full
resumption of Turkish military control over the Straits and the
refortification of the Dardanelles. Turkey was authorised to close the
Straits to all foreign warships in wartime or when it was threatened by
aggression; additionally, it was authorised to refuse transit from
merchant ships belonging to countries at war with Turkey. A number of
highly specific restrictions were imposed on what type of warships are
allowed passage. Non-Turkish warships in the Straits must be under 15,000
tons. No more than nine non-Turkish warships, with a total aggregate
tonnage of no more than 30,000 tons, may pass at any one time, and they
are permitted to stay in the Straits for no longer than three weeks. The
number of foreign warships permitted in the Straits at any one time is
restricted to one. Black Sea states are given more leeway, being
authorised to send capital ships of any tonnage through the Straits (but
only one at a time and specifically excluding aircraft carriers). They are
also allowed to send submarines through the Straits, with prior notice, as
long as the vessels have been constructed, purchased or sent for repair
outside the Black Sea. The less restrictive rules applicable to Black Sea
states were agreed as, effectively, a concession to the Soviet Union, the
only Black Sea state other than Turkey with any significant number of
capital ships or submarines.[7][8] The passage of civil aircraft between
the Mediterranean and Black Seas is permitted, but only along routes
authorised by the Turkish government.[9]