It has been a long time since I did a J4 India Crush, but I have done it about four times, so I do have a good feel it for it.
Now, having said that, it can not “technically” be stopped I think, but I believe the Allies can make the cost of Japan doing it from an amphibious assault too high. In the one game where I thwarted the attack here was my general game plan:
Bring the UK fleet and planes from the Med. and S. Africa to India. Build all naval units (carriers if possible) with India. Fly ANZAC planes to sea zone 39 and land on UK carriers. Move all French and ANZAC naval units into a combined Allied fleet by India. Now, this was several months ago, but my memory tells me that after round three, the Allies can have a defensive fleet roughly comparable to Japans offensive fleet (not including all of their land based planes, just units in the water). Japan can sink it, but, the cost would be very, very high. They would lose either a very large portion of their fleet or planes. In conjunction with this plan, I had the US build fleet in the Pacific for the first three turns. In my game, if Japan had chosen to go forward with the amphibious assault of India, the US fleet could’ve been able to run rough shod over the Pacific due to their fleet and airplane losses.
Problems with this Allied strategy:
Italy gets big immediately. The Allies are conceding the Med. to them from the start.
In my opinion, if Japan sees the UK do this, they might be able to simply go overland to take India.
Australia is an empty shell and Japan may have the option of taking them out instead of India.
Just my $.02 on the defensive plan to counter this.