Russo - Swedish War ( 1495 – 97 ) The Russo - Swedish War of 1495 – 1497 was a result of an alliance between Grande Prince Ivan III of Moscow and Hans of Denmark , who was waging war against the Sture family of Sweden in the hope of regaining the Swedish throne . It is believed that Hans promised to concede some stretches of Finnish lands to the Muscovite prince , although he did not bother to honour the agreement after he had been crowned King of Sweden at the close of the war . Pursuant to the agreement , Ivan III sent Princes Daniil Shchenya and Vasily Shuisky to lay siege to the Swedish castle of Viborg . The siege lasted for three months and ended when a castellan set his supply of powder on fire , thus " scaring the Muscovites out of their wits " , as the Swedish records say ( " see below for details " ) . The following year Russian generals Vasily Kosoy and Andrey Chelyadnin severely devastated Swedish Finland as far as Hämeenlinna ( Tavastehus ) . Another detachment sailed along the shore , forcing the Finns into subservience . Sten Sture the Elder , who was then at Turku ( Åbo ) , was enraged at the news of the Muscovite expedition and sent Svante Nilsson with 2 , 000 men to take Ivangorod , a new fortress which Ivan III had built to protect Russian Ingria against Livonian Knights . The fortress was taken without difficulty , but — as it was obviously impossible to defend it for a considerable period of time — Svante Nilsson proposed to hand it over to the Knights , an offer which they declined . Thereupon the Swedes set the fortress ablaze and sailed home . After the Swedish throne fell to Hans of Denmark , hostilities were suspended until 1508 , when Sweden and Moscow ratified a peace treaty for 60 years . Although the war yielded no tangible results to any of the belligerents , both countries corroborated the peace settlement in 1513 and 1524 . Explosion . The memory of the immense explosion in the Viborg Castle on 30 November 1495 survived in Finnish , and Russian , folklore . Probably some of the following stories are later inventions . A contemporary popular poem narrates the story of this blast . The defence of Viborg was led by its castellan , Lord Knut Posse . Muscovite invaders were just about to conquer the town . According to folklore , Knut Posse had caused the blast with some sort of exploding mixture . Other stories allege that the phenomenon was a figure shown against the sky and depicted for example the X - formed cross of St . Andrew . There are different opinions of the historicity and the real events of this phenomenon . There were only a couple of thousands soldiers on the Swedish side and defenders attempted to recruit peasants to add to their number . The first Muscovite attacks were successfully repelled , but in late November , the Muscovites attacked with all their forces . The battle seemed lost . The attackers had succeeded in reaching tops of walls and several towers were in their power . After that , something happened which caused Muscovites to retreat . One story narrates that Knut had a powder mine or tar barrels under one tower , which he detonated . Muscovites alive after the explosion would not have known what happened and escaped in great fear . Another story tells of Muscovites escaping because they became afraid of a light phenomenon on the sky . 30 November is the feast of St Andrew and they thought the saint was protecting the town , making them halt the attack .