The Russian Black Sea Fleet sank a mothballed cruiser in the inlet to Crimea's Donuzlav Lake on March 5, effectively blocking access to the sea from Ukraine's primary naval installation on the peninsula. Seven of the Ukrainian navy's estimated 25 ships are reportedly trapped in the port of Novoozerne.
This action serves several purposes for the Russian forces operating in Crimea. It provides a way, albeit and impermanent one, for Russia to free up military assets for other uses. The Kremlin's operational planners still want to deny Ukraine the ability to concentrate force as the crisis continues, so isolating Donuzlav Lake is thus a prudent move.
Moreover, sinking the ship may also give Russia a possible new bargaining option. Much of the present day Ukrainian navy was appropriated from the former Soviet navy after the break up of the Soviet Union in 1991. In time, Russia might choose to retake part of its "lost fleet," depending on how negotiations play out with Ukraine.
Russia seems content to keep the besieged ships at Novoozerne contained as bargaining leverage. Ultimately, the main goal for Russia appears to be keeping all Ukrainian forces in Crimea paralyzed while deterring any military operation that seeks to alter the status quo on the peninsula. Dividing the Ukrainian fleet achieves this neatly.
