He knew about war because he served in one and had friends who died in them unlike the chicken hawks running country last few admins.
This is a good read on Reagan's pragmatism.
http://www.foreignpolicy.com/articles/2010/06/07/think_again_ronald_reagan?page=full
How do you define "He served in one"...referrring to war?
Reagan spent WWII making training films for the military...he never served in combat in any way.
http://en.wikipedia.org/wiki/Ronald_Reagan
"Military service
After completing fourteen home-study Army Extension Courses, Reagan enlisted in the Army Enlisted Reserve[28] on April 29, 1937, as a private assigned to Troop B, 322nd Cavalry at Des Moines, Iowa.[29] He was appointed Second Lieutenant in the Officers Reserve Corps of the Cavalry on May 25, 1937,[30] and on June 14[citation needed] was assigned to the 323rd Cavalry.[not in citation given][30]
Reagan was ordered to active duty for the first time on April 18, 1942. Due to his nearsightedness, he was classified for limited service only, which excluded him from serving overseas.[31] His first assignment was at the San Francisco Port of Embarkation at Fort Mason, California, as a liaison officer of the Port and Transportation Office.[32] Upon the approval of the Army Air Force (AAF), he applied for a transfer from the Cavalry to the AAF on May 15, 1942, and was assigned to AAF Public Relations and subsequently to the 1st Motion Picture Unit (officially, the "18th AAF Base Unit") in Culver City, California.[32] On January 14, 1943 he was promoted to First Lieutenant and was sent to the Provisional Task Force Show Unit of This Is The Army at Burbank, California.[32] He returned to the 1st Motion Picture Unit after completing this duty and was promoted to Captain on July 22, 1943.[29]
In January 1944, Captain Reagan was ordered to temporary duty in New York City to participate in the opening of the sixth War Loan Drive. He was re-assigned to the 18th AAF Base Unit on November 14, 1944, where he remained until the end of World War II.[29] He was recommended for promotion to Major on February 2, 1945, but this recommendation was disapproved on July 17 of that year.[33] He returned to Fort MacArthur, California, where he was separated from active duty on December 9, 1945.[33] By the end of the war, his units had produced some 400 training films for the AAF.[29]"
OR, if you prefer a different link:
http://www.reagan.utexas.edu/archives/reference/military.html
"Ronald Wilson Reagan enrolled in a series of home-study Army Extension Courses on 18 March 1935. After completing 14 of the courses, he enlisted in the Army Enlisted Reserve on 29 April 1937, as a Private assigned to Troop B, 322nd Cavalry at Des Moines, Iowa. He was appointed Second Lieutenant in the Officers Reserve Corps of the Cavalry on 25 May 1937. On June 18 of that year Reagan, who had just moved to Los Angeles to begin his film career, accepted his Officers Commission and was assigned to the 323rd Cavalry.
Lieutenant Reagan was ordered to active duty on 19 April 1942. Due to eyesight difficulties, he was classified for limited service only, which excluded him from serving overseas. His first assignment was at the San Francisco Port of Embarkation at Fort Mason, California, as liaison officer of the Port and Transportation Office. Upon the request of the Army Air Forces (AAF), he applied for a transfer from the Cavalry to the AAF on 15 May 1942; the transfer was approved on 9 June 1942. He was assigned to AAF Public Relations and subsequently to the 1st Motion Picture Unit in Culver City, California. Reagan was promoted to First Lieutenant on 14 January 1943 and was sent to the Provisional Task Force Show Unit of This Is The Army at Burbank, California. Following this duty, he returned to the 1st Motion Picture Unit, and on 22 July 1943 was promoted to Captain.
In January 1944, Captain Reagan was ordered to temporary duty in New York City to participate in the opening of the sixth War Loan Drive. He was assigned to the 18th AAF Base Unit, Culver City, California on 14 November 1944, where he remained until the end of the war. He was recommended for promotion to Major on 2 February 1945, but this recommendation was disapproved on July 17 of that year. On 8 September 1945, he was ordered to report to Fort MacArthur, California, where he was separated from active duty on 9 December 1945."
So, while he DID serve in the military, it certainly didn't give him any special insight into the horrors of war. He spent most of WWII in Los Angeles and New York City.