Thanks Robert and MM
I have just confirmed his letter was edited and introduced to Japanese people in 1944.
His warning part is summarized as "I hope you would never make mistake in your unique policy against India".
A Japanese Buddhists group, Nipponzan-myohoji, tells a story -
Two Japanese monks of the group, Gyoryo Maruyama and Keisyo Tenzaki, were sent to India by the request of IJA in 1935. They stayed in Gandhi's Sevagram ashram of Wardha. Maruyama was arrested by the British Indian government in October 1940 to be deported because he had contacts with some Indian independence activists like Subhas Chandra Bose. Tenzaki kept staying as Gandhi's cook and handed his letter to IJA in mid-September 1942.
Need further careful research but I wonder if Gandhi's letter had discouraged the IJA officers and men in the region.
During the Battle of Imphal, Lt Gen. Kotoku Sato suddenly retreated his troops from the Indian border not obeying Commander Renya Mutaguchi's order. Mutaguchi got angry but not only Sato did not apologize but even his men refused to salute to Mutaguchi.
Sato was regarded insane and had been thrown into a mental hospital.