British and French plans for 1917 were agreed at an Allied conference at Chantilly from 1916. Existing operations were to continue over the winter, fresh troops arriving in front-line units were to be trained and in the spring the front of attack was to be broadened, from the Somme to Arras and the Oise. The front of attack was to be about long, with two French surprise attacks near Rheims and in Alsace, to begin after the main attacks, to exploit German disorganisation and lack of reserves. The Allies expected to have against divisions, for the co-ordinated offensives. A British operation in Flanders was also agreed, to begin several weeks after the main offensives further south. Joffre was replaced by Nivelle on 13 December, who proposed a much more ambitious strategy, in which the plan for a resumption of Anglo-French attacks either side of the Somme battlefield of 1916 was retained but the offensive on the Aisne was converted to a breakthrough offensive, to be followed by the commitment of a strategic reserve of to fight a "decisive" battle leading to the exploitation of the victory by all of the British and French armies. French troops south of the British Fourth Army were freed to join the strategic reserve by an extension of the British front, to just north Roye on the Avre facing St Quentin, which was complete by 26 February.