Operation Durga's Trishul
By the 1970's, the Indian Air Force had actually more or less become the most experienced, most modern air force in the entire Asian continent. With the Indian occupation of Sri Lanka increasingly loathed by locals, the Indian Army became increasingly reliant on air power to stabilize the front. This led to the increasing sophistication of Indian bomber capabilities, which only further developed as India purchased a very large chunk of America's decommissioned and surplus airpower. Although Operation Durga's Trishul was widely believed to have been planned in coordination with British and French forces, the Indians had actually rebuffed French assistance as unnecessary (the British had actually not offered assistance, contrary to public perceptions both in Britain and in South Asia).
Indian intelligence had understood that the Pakistani nuclear weapons program was far more advanced than originally believed. Although Western intelligence agencies dismissed the program as merely an East German vanity project, the actual driving force for the scientific development was Pakistani, chiefly the brilliant Abdus Salam. Believing that a war from NATO was imminent, the East Germans harried the Pakistanis to simply speed the project up. Instead of constructing a separate nuclear facility for uranium enrichment, the enrichment facilities simply became a side-annex to the Karachi Nuclear Power Complex. The Soviet military noted with rather deep unhappiness that despite not supporting this program in any way, the KNPC was literally just based on stolen designs of the new Soviet RMBK-1000 model, introduced only five months before construction on KNPC had begun, which led many scientifically-minded Soviets (otherwise well-predisposed to the present state of affairs in the USSR) to also start believing that the "Second Revolution" had seriously harmed the national security capabilities of the USSR.
Cognizant of the natural dangers of constructing nuclear tests in Pakistan's second largest city and capital (only Dhaka was larger), the Pakistani government was able to secure top-of-the-line anti-air defense capabilities from the Warsaw Pact and North China to defend the installation, which quickly became one of the most heavily defended sites on Earth (at least from aerial assault). The IAF had access to some of the most advanced bombers in the world (namely American-built F-111As), but was given the task of bombing the unbombable. An idea was floated by a surprising source, former Luftwaffe ace Hans-Ulrich Rudel, then-serving as a freelance advisor to both French forces in Cambodia and Indian forces in Sri Lanka. His proposal was simple: if you couldn't bomb the uranium enrichment facilities, then blow up the entire nuclear reactor next to the facility.
The problem with the design of Soviet nuclear reactors being pitifully easy to steal by the Pakistani ISIS meant that they were at least possible for the Western powers to steal. Having such plans, the Indians determined severe weaknesses in the design. On a stormy morning in June of 1971 (in the middle of monsoon season), a squadron of IAF planes took off to commence Operation Durga's Trishul. The ability to fly at near max speeds barely hundreds of feet above ground level (or building level) meant that the F-111As were able to evade most Pakistani anti-air defenses, while the monsoon weather scrambled the ability of the Pakistanis to respond. As a result, much to the shock of the Pakistanis, IAF bombers were able to deliver all of their payloads before being shot down (of which many were shortly after on the way back). Successive strikes on coolant and water pumping stations more or less guaranteed a meltdown. Multiple massive explosions ripped through the plant, further worsened by the monsoon conditions (which made it essentially impossible to drain water as needed).
Prime Minister Hasim ultimately made the fatal decision to not evacuate the Pakistani government from Karachi, even though an attempt to evacuate the civilian population was made. However, given Karachi's massive population (almost four million), it was deemed essentially impossible to evacuate most of the civilian population. However, that would not stop most civilians from fleeing the "Black Monsoon. of 1971." So much (radioactive) dust had been swept into the monsoon itself, the rains across almost the entirety of West Pakistan and Northwest India dripped black. Given widespread panic and flight of hundreds of thousands of civilians, it became quickly impossible to stop the reactor fire.
Civil order in West Pakistan, already under significant threat from the Safir Revolution, began unravelling. Paramilitary forces under the Indian-backed Jamaat-e-Islami Pakistan quickly dusted off old weapons, attacked government buildings, and specifically targeted Ahmadiyya Muslims (Abdus Salam, the head of the Pakistani nuclear program, was notably an Ahmadiyya Muslim, who were often seen as the strongest supporters of Pakistani socialism). Chaos on the streets of West Pakistan only further but erroneously convinced the the Pakistani leadership that an existential invasion from India was imminent.
In Bangladesh, unprecedented fury convulsed the population as hundreds of thousands of retired veterans and reservists quickly began answering volunteer organizations. Condemnation quickly rolled in against India from more or less the entire Eastern bloc and global South, with the Western bloc remaining palpably silent, with some even denying Indian involvement. Prime Minister Gandhi was taken back by the criticism, having been essentially ensured by most advisors and foreign diplomats that the international community would accept the rationale of destroying an illegal nuclear weapons program. However, it ended up that most of the world did not buy the Western narrative that the destruction of the Karachi Nuclear Power Complex was only an incidental "accident."
In the crucial next days, Prime Minister Gandhi largely spent her time coordinating humanitarian refugee programs at the Indo-Pakistani border, evacuating Indian residents from the effects of the Black Monsoon, and trying to make the case to non-aligned nations that the Karachi disaster was largely an incidental disaster caused by the Pakistanis intentionally placing their nuclear weapons program next to their largest nuclear power plant. These seemingly sincere efforts actually quieted a significant amount of international fury and outside of the Social Camp, calls for an anti-Indian embargo quickly died off. However, it did nothing to quiet the omnipresent calls for revenge in both West and East Pakistan and would be considered by some as a form of "dithering" before a Pakistani response that quickly exceeded even the wildest expectations of the Eastern bloc.
Ironically enough, despite the massive destruction and misery wrought by the Karachi disaster, the uranium enrichment facilities of the Pakistani nuclear power program were built under such formidable bunkers, they had actually mostly survived intact, even if significantly delayed.