Upon the Austro-Prussian War of 1866 and the break-up of the German Confederation in the Peace of Prague, the North German Confederation was established, instigated by the Prussian minister-president Otto von Bismarck. Originally a military alliance, it evolved to a federation with the issuing of a constitution with effect from 1 July 1867. In the course of the war, Prussian troops had occupied the Free City of Frankfurt and the King of Prussia (later to become Emperor of Germany) had purchased the remnants of the Thurn-und-Taxis Post organisation.