Editors. "Sweden enters the Thirty Years War". The Literary Encyclopedia. First published 01 February 2011
[http://www.litencyc.com/php/stopics.php?rec=true&UID=15978, accessed 18 April 2014.]
Sweden and Denmark sign a treaty undertaking the mutual defence of Stralsund against Imperial commander Wallenstein. By signing this treaty, King Gustavus II of Sweden is effectively brought into the Thirty Years War, from which until now he has stayed aloof.