Japanese plans had been on the table to invade India after the rapid conquest of Burma in 1942. However these plans were put off after the 1942 Guadalcanal campaign and the Japanese army remained on the defensive for a time.

In 1943, two things happened: first, the Chindit raids by British commandos began, causing relatively minor damage but becoming a propaganda coup for the Allies. Secondly, General Renya Mutaguchi became the head of the 15th Army, the main Japanese formation in Burma.

Mutaguchi was a bit of a strange fellow. He had been involved in Japanese operations in China (including the Marco Polo incident) and in Malaya and Burma. He had a low opinion of the British and Indian troops, and urged an attack on Assam (eastern India). This was rejected by many of his officers as being extremely foolish, as the Japanese defense line in the Arakan Mountains was quite solid, having repulsed an Allied attack earlier in the year. Mutaguchi presumably thought that this was a sign of weakness by the British and Indians and wanted to advance into Assam to shorten the defense line (allowing increased concentration of Japanese troops). There were also thoughts of perhaps bringing in the Indian National Army, a Japanese supported Indian independence organization, into Assam. Lastly, it is possible that Mutaguchi was contemptuous of the impending Allied counterattack and wanted to attack first to spoil their preparations.

Through his advocacy, combined with a lack of effective dissent (as several of his vocal opponents were dismissed and the others stayed quiet while expressing disagreement privately), he was able to convince the Japanese High Command to support his operation. His plan was to essentially supply his troops with a herd of animals that would be driven forward, and that soldiers were to pack heavy and be prepared to eat grass. This resulted in relatively ineffective troops, especially when one considered that they were forced to leave their heavier artillery behind in order to sustain the pace of advance, leaving them with few anti-tank options.

Needless to say, attacking in this manner was a terrible idea, resulting in extremely heavy casualties by the Japanese and weakening their forces in Burma enough to allow the Allied invasion timetable in Burma to be stepped up.

/u/Bernardito is more specialized in this theater of operations than I am, though, especially from the Allied perspective.