My Route: Moreh, Tamu, Witok. Kalewa. So I am now in the middle of the large no mans land which before Imphal separated the Japanese from the Allies. Some obvious things struck me.
First, is the side that defends ie the Allies pre-owns the roads it needs for the battle. So the Allies did all kinds of work on the various roads to where they intend to fight the Japanese eg Tiddim, Palel Shenam etc. Whereas the attacker ie the Japanese cannot get at the roads it will need till after the attack. Eg Kabaw Valley until after the attack. Super obvious point. So it was not so much supply through mountainous terrain that was the problem Slim did not want but supply across areas without roads which he could not get at in advance.
Kalewa where I was today is where the bulk of 33 Div crossed the Chindwin. Kalewa to Tamu took me 4 hours on a much improved road compared to 1944. There are about 53 small mostly Bailey bridges across chaungs which flood. About 5 of these had been washed away in the last year.
All to say I can understand the Japanese problem. Even on these two southern routes where they had the best pre existing roads there was nothing remotely suitable as a main supply route. As noted elsewhere the Japanese intelligence people had done a good job of knowing the ground but there is zero evidence this knowledge led to the engineering effort that would clearly be needed. In fairness Mutaguchi did ask for road building companies but was not given any.
I then went to a village which had been a Japanese supply hub and met and interviewed a 90 year old who had been there. They seemed to have got along just fine with the Japanese occupiers who mostly left them alone and were self sufficient in food.
Finally, Kalewa and the Japanese crossing site. I think the Chindwin was lower today than in March 1944. Nevertheless, it is not a tricky site. Wide lazy river. Decent approaches on drivable sandy beaches. The problem would have been monsoon in say May: river maybe twice as wide, flowing very fast so drag on pontoons needing some serious bank anchors. Centre of the pontoon bridge hard to control. AND most days the RAF come so it has to be taken apart and hidden. So you probably resort to ferries. Nightmare. You can quite see why supply at the front line was so poor.