He served as the Chief of the Office of Security Cooperation - Iraq (OSC-I). Before that, he served as the commanding general of the U.S. Army Combined Arms Center at Fort Leavenworth. In December 2009, Caslen was nominated to be a lieutenant general in conjunction with his nomination to be Commandant of the United States Army Command and General Staff College at Fort Leavenworth which took effect in March 2010. During Desert Storm he was executive officer of the 2nd Battalion, 187th Infantry, 101st Airborne Division (Air Assault).