Weeks after Mitsugi Murakami Kasai's brother returned from his Military Intelligence Service (MIS) duties in the Pacific War, Kasai decided to enlist in the Army. After completing the course at the MIS Language School in April 1947, he left for Counterintelligence Corps (CIC) training in Baltimore, Md. He completed his training and then taught Japanese for 10 months.

From June 1948 to June 1950 he served in the Tokyo/Kanagawa District under the 441st CIC Detachment, General Headquarters (GHQ), Far East Command. Working with local Japanese police departments, he kept track of Communist Party leaders and their activities. His activities reminded him of Federal Bureau of Investigation (FBI) and Office of Naval Intelligence operations that had "enabled the FBI to roundup the so-called 'dangerous' enemy aliens (Issei) for confinement in the Department of Justice internment camps." [Biography] As tension among the United States, China, and the Soviet Union rose, the CIC's roundup operations prepared to pick up members of Japanese Communist Party (JCP). 

The Air Force formed the Office of Special Investigations (OSI) at Tachikawa Air Force Base and Kasai went with Capt. Leo Carl to provide CIC coverage of the area. In July 1949, while Kasai was holding his first liaison party with all the police chiefs from the Santama District in Tachikawa City, one of the chiefs received an emergency call. A train had derailed killing six and injuring others and investigators suspected that Japanese Communist Party (JCP) members had caused the accident. Kasai followed the investigation of the 11 JCP members and reported to his headquarters. The evidence supported the conviction of only one of the 11 accused. Dubbed the "Mitaka Incident" it was one of many suspected communist plots the CIC tracked.

On June 16, 1950, Kasai was honorably discharged and returned to the continental United States with intentions of completing his degree in forestry. His plans changed when on June 25, 1950, the Korean broke out and he reenlisted as a Master Sergeant with the U.S. Army with the intention of joining the war effort. His plans were quickly derailed by his linguistic experience, as the Army asked him to study with the 11th Airborne Detachment in Kentucky, and then took a yearlong course in Cantonese. 

Instead of going to Korea, Kasai then moved to Niigata, Japan, with the CIC and monitored pro-Communist Korean organizations and Japanese who had been repatriated from Russia. It was not until 1953 that he got his wish to move to Korea where he monitored Chinese prisoners of war in a U.S. camp. 

Over the next 20 years Kasai moved back and forth across the Pacific Ocean serving in various intelligence operations in Japan, Okinawa, Korea, and Vietnam. He also served at the Presidio of Monterey, Calif., Fort Douglas, and other military installations throughout the United States. When he retired on March 31, 1973, he had been awarded the Meritorious Service Medal from the United States Army and letters of appreciation from the Koza Police Station in Tokyo and the Okinawa Prefecture Police.

