There Walker became a founding member of the Bloomington Guards, a local militia company, in late 1855, and he was quickly elected first sergeant.

In this capacity, Walker commanded the company at the Battle of Wilson's Creek, Missouri on August 10, 1861, where his regiment sustained over 50% casualties.

[2] In mid 1865, Major General Grenville M. Dodge ordered an expedition as a punitive campaign against the Sioux, Cheyenne and Arapaho.

Led by Brigadier General Patrick E. Connor, its strategy was to have three independently marching columns of soldiers converge upon the Sioux and Cheyenne in the Powder River Country.

The column started north from Fort Laramie in early August 1865 and traversed the country west of the Black Hills in Dakota Territory.

Marching into Montana Territory, the command suffered from bad water, weather, supply shortages, and harassing Indian attacks.