I have several friends who retired after 20 years and were recalled to active duty and ended up in Iraq and Afghanistan.
One of the more famous examples of recall:
Ted Williams interrupted his baseball career in 1943 to serve three years in the US Navy and Marine Corps for World War II. Upon returning to MLB in 1946, Williams won his first AL MVP Award and played in his only World Series. The following season he won his second Triple Crown. Williams returned to active military duty for portions of the 1952 and 1953 seasons in the Korean War, in which he served as a Marine aviator