India doesn't like a two front war with China and Pakistan in the North.
Bangladesh is not a factor for India. India will handle Bangladesh to it's own advantage in
any such war situation. In a Sino Indian or Sino-Pakistan Indian war Bangladesh can only support India or at best desperately try to be seen as "neutral".
Offering, or even appearing to offer any kind of diplomatic support to either Pakistan or China, will result in a crushing military response from India . Even neutrality will not be taken kindly by India, and India will demand, or take by force full road, rail, air and waterway transit across Bangladesh territory to support operations in Arunachal Pradesh.
It is far more likely that Bangladesh will offer these facilities on its own rather than risk an Indian military takeover.
For India, Bangladesh poses no risk to the "chickens neck" ( Siliguri corridor ). In case of an unforeseen collapse of the Indian front to China in that sector, India will swiftly invade Bangladesh to take over it's transit routes and facilities to support its operations in the east. This is exactly like Belgium's case in World War 2 when it's neutrality was meaningless in the face of Germany's military objectives.
Bangladesh's strategic and tactical situation is precarious. Bangladesh has a non-professional army with insufficient and outdated hardware riddled with corruption (
read Gen Hasan Sarwardy interview ).
At this time Bangladesh's Armed Forces have no capabilities of even protecting its citizens from wanton sniper fire by India's border guards.
The Bangladesh Armed Forces have no capabilities of protecting its own territory. The comments by PDF members here of Bangladesh taking over 7 Indian states by cutting the "chickens neck" are not realistic. Any such attempt will result in Indian retaliation that will cause severe or total destruction of Bangladesh's infrastructure.
Due to geography and the fact that Bangladesh is an enclave India doesn't need to put any boots on the ground or even commit its air force. Every bit of Bangladeshi territory is within range of Indian MRLS and its coastal access and ports is completely vulnerable to a Brahmos missile attack. India has allies within Bangladesh such as the Chakma insurgents. As a final punishment to Bangladesh India might carve out a "Chakmastan" from the Chittagong Hill Tracts and hand it over to the Chakmas.
If India turns on Bangladesh (in the course of a war) there is very little any foreign nation could do to help even if it wanted to. Unlike interventions in Syria or Afghanistan there is no big power vested interests in Bangladesh. Even China would be more concerned with taking Arunachal and defending Ladakh then bothering about Chittagong.
Indian war game planners have accounted for the scenario that in case
Bangladesh is annexed or even temporarily occupied there would be a partisan or gurilla resistance from remnants of the Bangladesh Armed Forces. This however would be handled by India far more easily than the situation faced by Pakistan in 1971.
India would not have the logistical disadvantages faced by Pakistan. The Indian occupation backed by India's 1 million paramilitary and reserve forces will outmatch Bangladesh numerically.( Example: West Bengals Eastern Frontier Rifles ( Bengali Speaking), supporting 26 battalions of the Bihar Regiment ( with many Bengali speaking assets) as well as the Bihar Military Police would be enough to secure Dhaka. India has huge Bengali language resources for its intelligence and population control which was unavailable to Pakistan in 1971.
The Bangladeshi resistance without shelter, protection and resupply from a friendly foreign base ( as in 1971) will quickly run out of resources. Bangladesh has no indigenous arms industry of any significance to support a people's resistance.
It is very likely that Myanmar ( looking over the shoulder at China ) will attempt a deal with India to annex the CHT.
What options does Bangladesh have ?
Obviously...
Keep marching in India's Republic Day parades, and hope that the BSF shoots less of its civilians each year.