Editors. "'Perpetual' Alliance between Prussia and Sweden". The Literary Encyclopedia. First published 01 April 2011
[http://www.litencyc.com/php/stopics.php?rec=true&UID=17072, accessed 21 April 2014.]
A 'Perpetual' Alliance is signed between Prussia and Sweden. According to the terms of this agreement, each promises to supply the other with military support in event of attack by a third party.