 Complete the following phrase:  The Allies frequently used crawler tractors in place of horses for hauling artillery and other supplies, the tracks often allowing them to negotiate terrain that was impassable to horse teams and wheeled vehicles. The British Army Service Corps also used them to haul long trains of freight wagons over the unimproved dirt tracks behind the front. They were particularly useful in October 1917, when Britain and France each sent six divisions to assist the Italians. Holt tractors ferried the supplies and ammunition over the steep and twisting mountain roads in less than two weeks. Two companies of Holt tractors with the dual role of gun-tractors and supply trains were also used in the 