After four years on the sidelines, Austria sought another war with France to avenge its recent defeats. Austria could not count on Russian support because the latter was at war with Britain, Sweden, and the Ottoman Empire in 1809. Frederick William of Prussia initially promised to help the Austrians, but reneged before conflict began. A report from the Austrian finance minister suggested that the treasury would run out of money by the middle of 1809 if the large army that the Austrians had formed since the Third Coalition remained mobilized. Although Archduke Charles warned that the Austrians were not ready for another showdown with Napoleon, a stance that landed him in the so-called "peace party," he did not want to see the army demobilized either. On 8 February 1809, the advocates for war finally succeeded when the Imperial Government secretly decided on another confrontation against the French.
Who was the Prussian ruler who at first  claimed he would help Austria in its battle with France, but later refused to?
Frederick William