Indolent, pleasure-loving and sensual, he was very different from his uncle and predecessor, Friedrich II 'the Great'. He was handsome and a patron of the arts, particularly music. A patron of Beethoven and Mozart, his own orchestra had a European reputation. As he was completely under the influence of his mistress, Wilhelmine Enke, his uncle used him for missions to other courts but was worried about the future of Prussia under this nephew. 
Friedrich Wilhelm II married his first cousin, Elisabeth of Brunswick, but this was not a success as Elisabeth, beautiful and high-spirited, refused to accept his infidelities and, after the birth of their daughter, took lovers herself. His uncle, worried that a bastard might inherit the throne, forced him to divorce her even though he was quite fond of her. She then went to live at Stettin but never remarried. 
However, Friedrich Wilhelm II did marry again and by his second wife had eight children. Nevertheless, there were again mistresses, two of whom he bigamously married "with the left hand" while his second wife was still alive and producing children herself. 
In 1781, still only heir to the throne, he joined the Rosicrucians and came under the influence of the fanatical Johann Christof Wollner. In 1786, when Friedrich Wilhelm II became king, Wollner was appointed privy councillor for finance and, except in name, became Prime Minister. 
Not approving of religious "enlighteners", proclamations were issued to protect religion from change. A new censorship law was issued and even a kind of Protestant Inquisition was established. 
As Friedrich Wilhelm II was no military man, he placed the army under both the Duke of Brunswick and General von Mollendorf, who through their neglect were the cause of Prussia's defeat at Jena in 1806. In 1787 he sent an army to Holland to assist his besieged sister, Wilhelmine, Princess of Orange; this proved a costly exercise which only delayed the inevitable. 
In 1793 and 1795, Prussia obtained extra territory from Poland. In 1791 during a meeting with the Emperor Leopold, he agreed to support the French King, Louis XVI, and a formal alliance was signed on 7 February 1792. Another treaty, on 19 April 1794 with the "sea powers", was financially advantageous for Prussia. However, the threat of Russia, made him sign a treaty with the French Republic on 5 April 1795, which was regarded as a betrayal by the European nations and as well left morally isolated. 
The unrest in the newly acquired territories were proving very costly and, when he died, Prussia was in a state of bankruptcy and confusion, with the army decayed and the monarchy discredited. He was only fifty-three when he died, worn out by his debaucheries
 

Source: Leo van de Pas