William I or Wilhelm I was King of Prussia from 2 January 1861 and German Emperor from 18 January 1871 until his death in 1888. A member of the House of Hohenzollern, he was the first head of state of a united Germany. He was de facto head of state of Prussia from 1858, when he became regent for his brother Frederick William IV, and he became king when his brother died three years later.
Under the leadership of William and his minister president Otto von Bismarck, Prussia achieved the unification of Germany and the establishment of the German Empire. Despite his long support of Bismarck as Minister President, William held strong reservations about some of Bismarck's more reactionary policies, including his anti-Catholicism and tough handling of subordinates. In contrast to the domineering Bismarck, William was described as polite, gentlemanly and, while staunchly conservative, more open to certain classical liberal ideas than his grandson Wilhelm II, during whose reign he was known as Wilhelm the Great.