First of all Turkey can issue to Russian ship a prohibition to pass through. If Russian ships nevertheless try to enter Bosphorus, Turkish ships from the three basis in and around the Sea of Marmara may use against Russia the same tactics Russia used while overrunning the Crimea: blocking the way of entrance with the ships. No shooting first and no aggressor status. And it would be not violating the Montreux convention:
https://en.wikipedia.org/wiki/Montre...of_the_Straits
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.
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