Claus was commissioned as a Second Lieutenant in the United States Army through the Reserve Officers Training Corps at Iowa State University in Ames, Iowa, in 1957.
He completed the Artillery Officers' basic course at Fort Sill, Oklahoma, in 1958 and later served on active duty at Fort Leonard Wood, Missouri.
In addition to artillery, he also became branch qualified in the infantry and in the quartermaster corps. Claus was promoted to First Lieutenant in July 1960.
He completed his military duty by serving in several Reserve and National Guard organizations, the last of which was the National Guard unit at Denison, Iowa.