Austerlitz incited a major shift in the European balance of power. Prussia felt threatened about her security in the region and, alongside Russia, went to war against France as part of the Fourth Coalition in 1806. One hundred and eighty thousand French troops invaded Prussia in the fall of 1806 through the Thuringian Forest, unaware of where the Prussians were, and hugged the right bank of the Saale river and the left of the Elster. The decisive actions took place on 14 October: with an army of 90,000, Napoleon crushed Hohenlohe at Jena, and Davout, commander of the III Corps, outdid everyone when his 27,000 troops held off and defeated the 63,000 Prussians under Brunswick and King Frederick William III at the Battle of Auerstadt. A vigorous French pursuit through Northern Germany finished off the remnants of the Prussian army. The French then invaded Poland, which had been partitioned among Prussia, Austria, and Russia in 1795, to meet the Russian forces that had not been able to save Prussia.