Idaho in the American Civil War The history of Idaho in the American Civil War is atypical , as the territory was far from the battlefields . At the start of the Civil War , modern - day Idaho was part of the Washington Territory . On March 3 , 1863 , the Idaho Territory was formed , consisting of the entirety of modern day Idaho , Montana , and all but southwest Wyoming . However , there were concerns about Confederate sympathizers in the eastern half of the territory , in what is present - day Montana . As a result , in 1863 Sidney Edgerton traveled quickly to see President Abraham Lincoln about the situation ; this was one reason to split the Montana Territory from the Idaho Territory . The split also resulted in most of Idaho Territory ' s land consisting of modern - day Wyoming being reassigned to the Dakota Territory . Volunteer soldiers who served in Idaho did not fight against the Confederacy , but instead monitored traffic along the Oregon Trail and protected communications routes between the western and eastern United States . Fort Boise , at the site of the city of Boise was founded on July 3 , 1863 . Old Fort Boise near the present day city of Parma was a French post ( thus where the name Boise comes from ) and was built in 1834 . it became one with the Snake River in 1854 , and although the French began to rebuild , they later abandoned it . Many people are confused by the two forts of the same name . Gold had been found at Orofino and Florence , Idaho that brought trouble with the friendly Nez Perce tribe . Camp Lapwai — later Fort Lapwai — was established to keep the peace between the Indians and the miners . The Bear River massacre took place on January 29 , 1863 , on what was thought to be the boundary of Washington Territory and Utah Territory near the present - day city of Preston in Franklin County , Idaho . On that date , 300 soldiers of the 3rd California Volunteer Infantry Regiment led by Colonel Patrick Connor slaughtered an encampment of at least 250 members of the Shoshone tribe ; only about twenty Shoshone men survived . This was in response to numerous attacks against white settlers in the previous year which were blamed on the Shoshone . The incident saw little coverage outside California and Utah , as most major newspapers were concerned with news of the main war fronts . The Bear River Massacre Site is currently a National Historic Landmark . There were some Southerners in the territory at the time . Most had originally gone to California , and came to Idaho to pursue gold mining . In the early part of the Civil War , Oregon and California Volunteer patrols had several clashes with the Paiute , Bannock and Shoshone bands in Oregon and the Territories of Washington ( later Idaho ) , Utah , and Nevada collectively known as the Snake Indians . However the invasion of their territory by miners in 1863 , to places like Silver City , brought on the Snake War . The Volunteers fought the Snakes until relieved by Federal troops in late 1865 ; the war continued until 1868 . After the Civil War , Idaho attracted many veterans of the Union Army to the state , including several governors . An Idaho Soldiers Home was constructed during the 1890s . In the 20th century , the deaths of veterans routinely made the news . The last Civil War veteran to die in Idaho was Israel Broadsword in 1952 .