Task: Question Generation
Input (English): The Germans expected to negotiate an end to the war but immediately ordered an advance on Paris; by 15 September Moltke issued the orders for an investment of Paris and on 20 September the encirclement was complete. Bismarck met Favre on 18 September at the Château de Ferrières and demanded a frontier immune to a French war of revenge, which included Strasbourg, Alsace and most the Moselle department in Lorraine of which Metz was the capital. In return for an armistice for the French to elect a National Assembly, Bismarck demanded the surrender of Strasbourg and the fortress city of Toul. To allow supplies into Paris, one of the perimeter forts had to be handed over. Favre was unaware that the real aim of Bismarck in making such extortionate demands was to establish a durable peace on the new western frontier of Germany, preferably by a peace with a friendly government, on terms acceptable to French public opinion. An impregnable military frontier was an inferior alternative to him, favoured only by the militant nationalists on the German side.
Output (English): On what date was the encirclement of Paris complete?