Operation Chengiz Khan
The strategic axis of an all-out war in South Asia never favored the Pakistanis. East Pakistan, or Bengal, was surrounded on all three sides by India. Indian troops were arrayed on each side of the border, ready to engulf the Communist stronghold in the outbreak of any war scenario. The Pakistani military always concentrated the bulk of their forces in West Pakistan, where the bulk of Pakistan's territorial disputes with India were (such as, but not limited to the Kashmir crisis that sparked the original war between the two). However, West Pakistan was in many ways a mess. The Communists had always been fairly unpopular in West Pakistan, with millions of refugees fleeing bouts of revolutionary violence that exploded. Operation Durga's Trident had ground daily life in the West to a halt - with most of the calls for revenge coming from the East.
Prominent Pakistani exiles in India, such as the former general Ayub Khan, informed his Indian supporters that the only way to defend East Pakistan would be in West Pakistan, an understanding which slowly permeated the entire armed forces of India. After all, the numerical superiority of India's armed forces seem so overwhelmingly, only in the West could it face a defeat based on tactical failure. Thus, the impetus was created to avoid such tactical failure. As it was understood, as long as they could hold the line in the West, East Pakistan would eventually fall, causing the regime to sue for peace. Significant Indian artillery and infantry formations were moved to the border with East Pakistan, with the doctirne emphasizing slow deliberate advances with artillery support in order to try to pulverize Pakistani army formations outside of cities - before slowly advancing into those cities. The extremely dense population of East Pakistan clearly necessitated these tactics.
The problem of course, is that the Pakistan which most Indians remembered had long since ceased to exist. Almost a decade had passed since the start of the Sifar Revolution. Estimates of the death toll varied, but it was generally understood to be in the hundreds of thousands. The olds had been smashed from statues, to old palaces, to even historical architecture. Even Urdu itself had been smashed. Being associated with "Indo-Aryan imperialism", Urdu was romanized and saw several Persian influences removed, which ironically brought it much closer to Hindi, something Pakistani authorities furiously denied even though it was true. The repeated governmental intrusions into Urdu made it relatively unpopular to many (since saying it in the wrong way opened people up to reprisal) - which ironically made English the de facto lingua franca of most West Pakistani elites. Bengali saw a rather lambasted romanization movement pushed by young radicals, but it was generally harder to say it in the "wrong way". Furthermore, demographic shifts (more refugees left West Pakistan than East Pakistan) quickly made Bengali the dominant language of Pakistan, alongside English.
This general program was more popular in East Pakistan than the West for many many reasons, something which Indian planners did not quite catch. Furthermore, with the bulk of the population in the East, the Pakistani popular militias were much much much larger in the East. Although the Pakistani Army was somewhat more numerous in the West, the popular militias were so numerous in the East, that the Pakistanis had in fact significantly more troops in the East in a significantly smaller territory.
Furthermore, the Pakistanis were receiving far more foreign tutelage than expected. One of the benefits of the joint nuclear program was that it also became a pretense for the nations of the Warsaw Pact, generally too scared to engage in air drills in actual Europe, to basically spend their time in Pakistan training. The last major war which saw significant air assets on both sides was the Three Years War - and that was a semi-limited war that saw relatively few attacks on air bases themselves. The North China-Israel war saw little such combat - nor did the UK-South Greece war. The Indonesian War and Burma War saw pretty one-sided aerial theaters. And so on and so on. As such, it was not as well understood how vulnerable unprotected air bases could be.
On a sleepy morning in 1968, waves of Pakistani MiG-21 and Sukhoi-Su-7BMs crossed into India - not from West Pakistan as widely expected, but urban airfields in hyper-densely populated East Pakistan and much to India shock - from Burma - in either case often cleverly disguised by civilian buildings. Significant Indian bomber assets had been moved from West Pakistan and even Ceylon to East Pakistan to bomb Communist revolutionaries to smithereens (which they did fairly effectively), but Indian fighter coverage was still heavily slanted in the West. The result was catastrophic. What few Indian fighters managed to make it into the air were knocked out almost immediately, as East German-produced airfield destroying bombs rendered most airfields in Eastern India inoperable. Bombers on the ground were blown up on the ground - and whatever few managed to make it into the air rarely lasted long.
In the span of roughly two days, wave after wave of SU-7s peppered air fields in Eastern India, destroying hundreds of planes (one estimate up to 600 airplanes), representing a large majority of the entire Indian Air Force, and essentially rendering inoperable every major air base in Eastern India. By the end of the second day, it was assessed that the Pakistani Air Force had achieved total superiority over Bengal and Northeast India, causing the Pakistanis to start targeting every Indian railway connecting Northeast India to the rest of the country. Although the rail hubs in Siliguri survived, the rails around the fortress did not. Communist revolutionary groups, hunted down by India's expert anti-guerilla operations, cheered as bombers came to their rescue.
The Indian Army had mustered around 360,000 men to surround the borders of East Pakistan. In contrast, the East Pakistanis only had around 130,000 regular soldiers (out of a total Pakistani army of around 290,000), but they were supported by around 130,000 popular militias, some of which had received significant training in Burma. The coup de grace came when due to complicated factional politics in the increasingly labyrinthine world of Burmese political struggles, the more moderate white flags encouraged the more adventurist red flags to join in the war simply to get rid of them.
Obviously the borders were significantly guarded, but the main concentration of troops was clearly organized in preparation for an offensive into East Bengal. Furthermore, significant assets were tired down in combatting both Communist rebels as well as various ethnic rebels in regions like Nagaland, Tripura, and Mizoram. Although most of the rebels were non-Communist, they had a line of communication with the Communists in Burma as they were their primary arms suppliers through the Burmese jungle.
Although Indian army troops rapidly switched to the defensive and prepared the best they could, the situation became quite daunting. The Pakistanis had total air superiority, Burmese militants flooded in from the border into a much more weakly defended back in cooperation with various ethnic militias, their armies had no viable supply lines, and they had mere hours to quickly switch to a defensive posture, build secondary lines, and entrench themselves before the Pakistanis assaulted. Indian army concentrations on the Western border of East Pakistan were able to supplied to some extent, but the other groupings alongside the northern and eastern borders were put in one of the worst positions a major army grouping had been put in since the Second World War - and superior Indian training held back relatively poorly trained Pakistani militias until ammo ran dry (supply by the sky and by the seas was also impossible - and the situation in Northeast India was essentially non-salvageable). The total collapse of the army corps in those sectors was inevitable as soon as regular Pakistani forces joined the fray (which they did late, to minimize losses.) Within a week, one after another, Indian army corps trapped in Northeast India surrendered, totaling somewhere between 250,000-300,000 soldiers - or almost a third of the entire Indian Armed Forces.
All in all, India retained significant numerical superiority over Pakistan - but those numbers were disproportionately tied up in West Pakistan - or Ceylon. In West Bengal, the numerical superiority had fallen to overwhelmingly favor Pakistan. This was unsurprisingly unanimously seen as a catastrophe in India, the second of three great disasters to befall India that year. Shock fell upon the Indian war cabinet as several generals urged the Prime Minister to desperately shift troops from Ceylon and West Pakistan to the east. She approved the request for Ceylon (leaving only barely enough to hold the line) - but to their shock, denied their request with regards to West Pakistan. They would be given another order entirely -
to advance. They had prepped an offensive for too long to simply throw it away to defend the east. The Prime Minister simply told the generals to make do in the east. She had seen thousands of West Pakistani refugees flood into India by the day - and had long since decided to end the bloody Marxist experiment to the west, an extremely fateful decision for both nations.
In the capitals of the West, diplomats and strategists were in dismay over this outcome, but not inclined to panic. It was a disastrous start to a war, but in their view, the Indians would be paying the price, so they simply didn't care. Furthermore, they were confident superior Indian numbers
and materials would grind down the Pakistanis in a matter of time. In some sense they were right...but in others, they were not.