The Prussian Province of Saxony was formed in 1815 from the territories, about 8100 sqmi in extent, ceded by the Kingdom of Saxony, with the addition of some districts already belonging to Prussia, the most important of which are the Altmark, from which the State of Prussia sprang; the former immediate principalities of the Archbishopric of Magdeburg and of the Bishopric of Halberstadt, which Prussia had received by the Peace of Westphalia (1648) at the close of the Thirty Years' War; and the Eichsfeld, with the city of Erfurt and its surroundings.