Editors. "Sweden agrees to aid Tsar Basil Shuisky of Russia against Poland". The Literary Encyclopedia. First published 01 February 2011
[http://www.litencyc.com/php/stopics.php?rec=true&UID=15662, accessed 16 April 2014.]
An alliance is signed at Viborg between Charles IX of Sweden and Tsar Basil Shuisky of Russia. This is an alliance on two fronts: the Tsar needs Sweden's assistance to quash False Dmitri II, who is attempting to claim the throne, while both are threatened by Sigismund III of Poland, who is advancing towards the Russian city of Smolensk.