The new VIII Corps , under the overall command of Édouard Mortier , included three infantry divisions and a division of cavalry ( see Order of Battle below ) . Corps Mortier , as it was known , crossed the Danube at Linz and Passau in early November 1805 and marched east , on the north bank of the Danube . Operating independently , the corp 's cavalry conducted reconnaissance ahead of them and on the flanks . Gen. Gazan 's division ( about 6 @,@ 000 men ) took the lead ; Mortier was with them . They were followed by Gen. Dupont 's division ( another 4 @,@ 000 ) about one day 's march behind . Jean @-@ Baptiste Dumonceau 's division ( another 4 @,@ 000 ) , marching another day behind Dupont , brought up the rear . A flotilla of 50 boats acquired at Passau provided communications across the Danube . Before sending Mortier on his mission , Napoleon instructed him to protect his north flank at all times against possible Russian reinforcements , advice he reiterated in subsequent written orders . Napoleon also advised Mortier to secure all crossings of the Danube between Linz and Vienna .