On March 4, 1931, a telegram sent by Kasahara to the general staff in Tokyo was intercepted and decoded by Soviet military intelligence and forwarded to Stalin.

On December, 13, 1931, the OGPU decoded and forwarded to Stalin a conversation between Kasahara and his superior visiting Moscow, which advocating for war before the USSR became too strong and underscoring that “the countries on the Soviet western border (i.e., at a minimum Poland and Romania) are in a position to act with us.

The thinking of Japanese ambassador to USSR, Hirota Koki, “the cardinal objective of this war must lay not so much in protecting Japan from Communism as in seizing the Soviet Far East and Eastern Siberia”, was also mentioned.

On his return to Japan, he was assigned to the Soviet Branch of the 4th Section (European & American Military Intelligence), 2nd Bureau, of the Imperial Japanese Army General Staff.

However, in April 1945, he was appointed to replace General Yoshio Kozuki as commander of the IJA 11th Army, and thus participated in the Operation Ichi-Go offensive, notably at the Battle of Guilin-Liuzhou.