Question: What mountain range was crossed by the wagon road that connected Fort Walla Walla to the fort established in 1846 in Montana?
Context: Mullan Road was the first wagon road to cross the Rocky Mountains to the Inland of the Pacific Northwest.  It was built by U.S. Army troops under the command of Lt. John Mullan, between the spring of 1859 and summer 1860.  It led from Fort Benton, 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).  The road connected to Fort Walla Walla, Washington Territory, near the Columbia River.  The road previewed the route approximately followed of modern-day Interstate 15 and Interstate 90 through present-day Montana, Idaho, and Washington. Fort Benton is a city in and the county seat of Chouteau County, Montana, United States.  Established in 1846, a full generation before the U.S. Civil War, Fort Benton is one of the oldest settlements in the American West; in contrast, many other places—including large cities today—were settled in the late 1860s, 1870s, or 1880s.  The city's waterfront area, the most important aspect of its 19th-century growth, was designated the Fort Benton Historic District, a National Historic Landmark, in 1961. The Macleod-Benton Trail was a wagon road that connected Fort Benton in Montana to Fort Macleod, Alberta on the Canada–US border.  It was also known as the Whoop-Up Trail. Fort Walla Walla is a United States Army fort located in Walla Walla, Washington.  The first Fort Walla Walla was established July 1856, by Lieutenant Colonel Edward Steptoe, 9th Infantry Regiment.  A second Fort Walla Walla was occupied September 23, 1856.  The third and permanent military Fort Walla Walla was built in 1858 and adjoined Steptoeville, now Walla Walla, Washington, a community that had grown up around the second fort.  An Executive Order on May 7, 1859 declared the fort a military reservation containing 640 acres devoted to military purposes and a further 640 acres each of hay and timber reserves.  On September 28, 1910 soldiers from the 1st Cavalry lowered the flag closing the fort.  In 1917, the fort briefly reopened to train men of the First Battalion Washington Field Artillery in support of action in World War I.  In 1921, the fort and property were turned over to the Veterans Administration where 15 original buildings from the military era remain.  Today, the complex contains a park, a museum, and the Jonathan M. Wainright Memorial VA Medical Center. The Santiam Wagon Road was a freight route in the U.S. state of Oregon between the Willamette Valley and Central Oregon regions from 1865 to the 1930s.  It is considered one of the most important historical routes in the state.  Officially known as the Willamette Valley and Cascade Mountain Wagon Road, the Santiam Wagon Road was built between 1861 and 1868 and operated as a toll road until 1915.  In 1925, the road was turned over to the State of Oregon for use as a highway.  U.S. Route 20 closely follows the original route of the wagon road. The Battle of Toppenish Creek was the first engagement of the Yakima War in Washington.  Fought on October 5, 1855, a company of American soldiers, under Major Granville O. Haller, was attacked by a band of Yakamas, under Chief Kamiakin, and compelled to retreat.  The battle occurred in Yakima Valley, 113 miles northwest of Fort Walla Walla, along Toppenish Creek and was a major victory for Native American forces. The Fort Walla Walla–Fort Colville Military Road was built in June 1859 to connect the Walla Walla area with its fairly easy access to the Columbia River to the mountainous area of the Huckleberry and Selkirk Mountains of current Northeast Washington and the Inland Northwest.  Brigadier General William S. Harney, commander of the Department of Oregon, opened up the district north of the Snake River to settlers in 1858 and ordered Brevet Major Pinkney Lugenbeel, 9th Infantry Regiment (United States) to establish a U.S. Army post to restrain the Indians lately hostile to the U.S. Army's Northwest Division and to protect miners who flooded into the area after first reports of gold in the area appeared in Western Washington newspapers in July 1855.  It was common practice to use existing Indian trails to develop military roads, and only make necessary improvements for the movement of artillery or supply trains.  Brevet Major Lugenbeel followed the long established Indian trail, then Hudson Bay Company brigade trail from the U.S. Fort Walla Walla area to Hudson Bay Company Fort Colvile, but had to leave the trail at current Orin-Rice Road, two miles south of Colville, where the southernmost land claims of the Hudson Bay Company fort began.  Washington Territorial Governor Isaac Stevens and the U.S. Army were ordered by the United States Department of State to honor land ownership claims by the Hudson Bay Company.  Lugenbeel's command arrived from Fort Walla Walla on June 20, 1859; Pinkney Lugenbeel was later credited with improving and building up the Military Road. The Whitman massacre (also known as the Walla Walla massacre and the Whitman Incident) was the murder of Oregon missionaries Marcus Whitman and his wife Narcissa, along with eleven others, on November 29, 1847.  They were killed by a party of Cayuse Native Americans who accused him of having poisoned 200 Cayuse in his medical care.  The incident began the Cayuse War.  It took place in present-day southeastern Washington state, near the town of Walla Walla, and was one of the most notorious episodes in the U.S. settlement of the Pacific Northwest.  The event was the climax of several years of complex interaction between Whitman, who had helped lead the first wagon train to cross Oregon's Blue Mountains and reach the Columbia River via the Oregon Trail, his wife and fellow missionary Narcissa, and the local Native Americans.  The story of the massacre shocked the United States Congress into action concerning the future territorial status of the Oregon Country.  The Oregon Territory was established on August 14, 1848. The Dalles Military Road, also known as The Dalles – Boise Military Wagon Road, was a mid-19th century wagon road surveyed and barely built by The Dalles Military Road Company between 1868 and 1870.  To qualify for government land grants, the company was supposed to build a wagon road from The Dalles, Oregon to Fort Boise in Idaho.  However, the company's road, on which it spent about $6,000 and for which it received nearly 2300 km2 of public land, consisted largely of existing wagon roads and rudimentary trails.  In particular, the company took credit for building a well-traveled and pre-existing wagon road between The Dalles and Canyon City, Oregon. Fort Nez Percés (or Fort Nez Percé, with or without the accent), later known as (Old) Fort Walla Walla, was a fortified fur trading post on the Columbia River on the territory of modern-day Wallula, Washington.  Despite being named after the Nez Perce people, the fort was in the traditional lands of the Walla Walla.  Founded in 1818 by the North-West Company, after 1821 it was run by the Hudson's Bay Company until its closure in 1857.
Answer: Rocky Mountains