He graduated from the United States. Military Academy at West Point in 1975 and played center for the Army Black Knights football team He also earned a master"s degree in business administration from Long Island University and a Master of Science degree from Kansas State University.
He served as the Chief of the Office of Security Cooperation - Iraq (OSC-I). Before that, he served as the commanding general of the United States. 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).
He was brigade operations officer, 3rd Brigade, 101st Airborne Division (Air Assault). J-3 in Honduras for Joint Task Force Bravo in 1998.
Executive officer to the deputy commander in Haiti during Operation Uphold Democracy in 1994–1995. Commander of the 1st Battalion, 14th Infantry, 25th Infantry Division (Light).
Senior brigade C2 observer/controller, Operations Group, Joint Readiness Training Center.
Chief of staff of the 101st Airborne Division. And commander of the 2nd Brigade, 101st Airborne Division (1999). He was at the Pentagon during the September 11 attacks where he was an assistant deputy director of strategic planning and policy.
He helped search for wounded and also worked on the air conditioning system following the attack.
He was also chief of staff of the 10th Mountain Division (Light) (2003) at Fort Drum. Assistant division commander (Maneuver) of the 3rd Infantry Division (Mechanized) (2004).
Deputy director for the War on Terrorism, J-5, the Joint Staff (2005 – June 2006). Director of the Joint Staff"s Detainee Affairs Division and commandant of cadets for the Military Academy at West Point, a post he left in May 2008.
From May 2008 to December 2009, he served as commander of the 25th Infantry Division (United States).
In October, he returned from a tour with the division in the Iraq War. Prior to this, he had served two tours in the War in Afghanistan (2001–present) and one tour in the Gulf War. Caslen was chief of staff for the Combined Joint Task Force 180 (CJTF-180) in Afghanistan from May through September 2002.
He was a commander of Operation Champion Sword.
According to a report by the Office of Inspector General, Caslen "violated the ethical guidelines of the military that forbid officers to promote private organizations" when he appeared in a video for an evangelical organization, Christian Embassy. Other generals that appeared in the video included Vincent K. Brooks and Air Force generals Peter U. Sutton and Jack J. Catton Junior.
At the time he was Commandant of Cadets at West Point. In a letter, he said he would be more vigilant about such improprieties going forward.
In July 2011, he was nominated to be chief of Office of Security Cooperation in Iraq.
7 Overseas Service Bars.