The 51st Armoured Division is a division of the UNSC Army Corps. Originally stationed at Fort York on the surface of Mars the 51st was meant to defend Mars in case of a covenant invasion. Before the creation of the 51st its members were UNSC Army Corps members that had participated in various battles with the covenant. The 51st was formed in 2550. In 2552 it was assigned to the UNSC Second Assault Fleet.