King Frederick William III of Prussia.
Frederick William III of Prussia (1770 - 1840) was King of Prussia from 1797 until his death in 1840 and was the last Elector of Brandenburg from 1797 until the abolition of the Holy Roman Empire in 1799. He was also King Frederick William I of the Germanic Confederation.
Frederick William came to his throne after the death of his father Frederick William II during the Habsburg War, with Prussia and Great Britain locked in a stalemate with France and Austria. Within a year of his accession, the Prussians and their British allies had broken the stalemate and defeated the French and Austrian armies. In 1799 Frederick William took part in the negotiations for the Treaty of Aix-la-Chapelle that ended the war. Under the terms of the treaty, Prussia annexed Austrian territory (possibly the Kingdom of Bohemia and the Margraviate of Moravia) and united with various allied states to form the Germanic Confederation.
Frederick William continued to rule the Germanic Confederation until his death in 1840. Sobel implies that another war between the Germanic Confederation and Austria took place in the latter half of the 1810s, most likely ending in another German victory.
Frederick William III does not have an entry in Sobel's index. He is mentioned on p. 62 of For Want of a Nail ....