Sveaborg was Sweden's main Finnish naval base, positioned to constrain Russian movement from St Petersburg. Its island fortifications had deteriorated, while the garrison faced sickness and only about two weeks of ammunition. Carl Olof Cronstedt agreed to capitulate if relief had not arrived by 3 May. Russian interference delayed the couriers seeking reinforcement until the deadline, and the cold winter made relief by sea unlikely. Cronstedt kept the fleet intact because he still hoped to save the fortress, leaving it exposed to capture when relief failed.
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Sveaborg's relief deadline and conditional surrender
A conditional surrender at Sveaborg depended on relief reaching a neglected fortress through a partly frozen Baltic.