Before Prussia and Russia could join their forces, Napoleon’s army crushed a Prussian and Saxon army under the Duke of Brunswick at Jena in Saxony. The victory enabled Napoleon to occupy Prussia before taking on Russia, and thus to enforce the Treaty of Tilsit in 1807.
Editors. "Battle of Jena and Auerstadt". The Literary Encyclopedia. First published 01 February 2010
[http://www.litencyc.com/php/stopics.php?rec=true&UID=4401, accessed 01 May 2016.]