On 21 July 1536 du Bellay was nominated "Lieutenant-General" to the king at Paris and in the Île de France, and was entrusted with the organisation of the defence against the Imperialists under the leadership of the Count of Nassau, who, under the direction of the Emperor Charles V, were invading eastern France while Charles was attacking Provence. When his brother Guillaume du Bellay went to Piedmont, Jean was put in charge of the negotiations with the German Protestants, principally through the humanist Johannes Sturm and the historian Johann Sleidan.