In WW2 there was a Japanese soldiers’ saying: Java was Heaven; Burma was Hell, but from Guinea you never came back.
Burma was a relatively small operation and about 10 times more Japanese soldiers died in Guinea. (Guinea was the route to Australia which Japan had originally wanted to invade) In March 1944 the bulk of the Japanese Army was in China. But Burma was the north east flank of a defined strategic Circle of Defence. This was defined in 1943 when things started to go badly for the Japanese. So it was not that India had to be invaded, but Burma had to be held.
However, Japanese doctrine learned from the Germans, did not really allow for defence. As soon a commander saw he was about to be attacked, the response was to attack somewhere pre-emptively, So, as soon as it was clear the British were planning to re take Burma, Japanese doctrine called for an offensive to pre-empt it. Doctrine aside, Burma was a vast area to defend and the number of Japanese troops too few to hold it. An attack was an obvious course of action.
U Go which was the Japanese name for their offensive operation suffered from being 2 plans in one operation. Commanders most certainly did not Select and Maintain the Aim. The commanding General Renya Mutaguchi, wanted to invade the whole of India, Almost everyone else on the Japanese staff knew this was impossible and so agreed to the operation on the basis that if it petered out even a few kilometres over the Chindwin, then their defensive position would be improved. Of course, getting Imphal itself was the stated aim since the road went through the town which was therefore key to movement in either direction. I will save for the book but there is a key bit of evidence that showed Mutaguchi wanted to invade India, everyone knew it, but no one stopped him. The blame for U Go goes far wider than Mutaguchi.
1943 had been a miserable year for the Japanese Burma Area Army. They had built roads, occupied but not controlled huge swathes of Burma, food and supplies were rare. There was not much to do besides chasing elusive Chindits who went, it seemed, wherever they pleased. Japanese morale was rock bottom.
Mutaguchi had plenty of bad luck. His divisional commanders were all bright top drawer squeaky-trousered staff officers – less Sato who went to Kohima who had combat experience. Lt Gen Yanagida (33 Div) who came up the Tiddim Road was repelled by the sight of the wounded and dead. Lt Gen Masafumi Yamauchi who came through Ukhrul bought his own western lavatory and was himself fatally ill on arrival. He was a graduate of US staff College at Fort Leavenworth Kansas. (I do want to pull his record from the Archive) The Infantry Commander of 33 Dvision was Maj Gen Tsunoru Yamamoto who lead the Tamu – Moreh, Pallel Axis. He was determined experienced and his performance showed it. The others were all against the U Go plan in the first place and said so. Lt Gen Sato, rather than give a rousing speech to troops on the eve of battle warned his division of high commands incompetence. Command dysfunction was a feature of the Japanese command group. Sato, Mutaguchi and Kawabe Commander Burma Area Army, all had a fascinating prior relationship.
15th Dvision which was to advance in the the centre was held back in Siam building roads, incredibly, partly to sabotage the plan. They arrived late and poorly equipped. Again, the blame for the failure of U go goes wider than Mutaguchi.
The other important context was China which was central to both Allied and Japan strategy. In March 1994 the Japanese were already thinking about Operation Ichi -Go, although it did not start until later on. This was a largely successful drive down the East Coast of China to capture bases and ensure that US bombers could not reach the Japanese mainland. The Japanese had seen what strategic bombing had done to Germany and in 1944 this was their chief fear. Of course, a success in Burma would allow a hook up with Ichi -go forces heading south. Ichi go was much larger operation than U go.
Finally, it is right to note that by 7 March 1944 the Japanese were losing the War and losing it badly. Shipping loses were high. It was difficult to get spares, efforts to create ammunition production around Rangoon had not been successful. Everyone had less than they needed. But it was argued, none of this mattered too much since the Japanese soldier had unique fighting abilities and could prevail if given the chance.