Mullan Road was the first wagon road to cross the Rocky Mountains to the Inland of the Pacific Northwest. It was built by US Army Lieut. John Mullan between the spring of 1859 and summer 1860. It led from Fort Benton, Montana, which at the time was in the Dakota Territory, then Idaho Territory from July 1863, and Montana Territory beginning in May 1864, and the navigational head of the Missouri River (and once farthest inland port in the world) to Fort Walla Walla, Washington Territory, near the Columbia River.