In 1806 the tenuous alliance between Prussia and Napoleonic France became increasingly strained by French expansionist policies and Prussia declared war in October, beginning the War of the Fourth Coalition. Except for its ally Saxony, Prussia was alone in its fight against France during this opening phase of the war. Large numbers of French troops were still positioned in southern Germany from the previous year’s War of the Third Coalition, so Napoleon was able to quickly mobilize a large army to move north in three columns against the predicted formation of the Prussian line in Saxony. On 14 October, the separate French columns encountered the Prussian army at the twin battles of Jena and Auerstädt, during which the disorganized, cumbersome Prussian army was routed by the significantly outnumbered French. Following this decisive victory, Napoleon was able to push through Prussia with ease, securing Berlin on 27 October and an alliance with Saxony on 11 December.