THAT was the battle that wasn't. With nearly 30,000 heavily armed Pakistani troops entrenched in Dacca and their commander swearing to fight to the last man, the battle for Dacca this week was expected to be brutal. As the Review's Islamabad correspondent points out in the following story, the Pakistani government even gave up plans to prepare its people for the eventual loss of the eastern wing. President Nixon's "gunboats" in the Bay of Bengal perhaps kindled Pakistani hopes of an eleventh- hour miracle in the east.

What finally happened at the eleventh hour was a different sort of miracle: the triumph of military commonsense over political fanaticism. General A.A.K. "Tiger" Niazi's surprise request for a ceasefire was made on Wednesday night and it was reported he was acting under President Yahya Khan's instructions. In retrospect, it appears certain that a major split in the Pakistani leadership in Dacca forced the decision.

The split surfaced three days earlier, on Sunday, when General Rao Farman Ali, second in command in Dacca, approached the United Nations with surrender terms. Islamabad disowned the move, which only widened the Dacca split. On Tuesday the civilian government of A. M. Malik resigned en masse and asked senior civil servants to give up their posts. All of them proceeded to seek refuge in the neutral zone of the Intercontinental Hotel. With the civilian government publicly dissociating itself from the actions of Islamabad and top military leaders in no mood to fight a hopeless war, Pakistan's decision makers finally saw the game was up.

The point that ultimately prevailed in Dacca must have been that the alternative to an honourable surrender was the annihilation of some 70,000 professional soldiers and several thousand non-Bengali irregulars and civil servants. The Indian Army chief had repeatedly driven home the point by reminding the Pakistani commanders of the vengeful mood of the Mukti Bahini guerillas. By taking the hint, the Pakistani commanders have avoided unnecessary blood letting.

The Indians are believed to have entered into a pact with the Bangla Desh leadership about the withdrawal of their troops as soon as the Pakistani forces have been defeated. This squares with Prime Minister Indira Gandhi's reported determination to lean over backwards to demonstrate that India will not interfere in the affairs of Bangla Desh.

The Indians will have no justification now to continue the war on the western front. But more important is the question of what to do with the Pakistani troops in the east. India is committed to treating them according to international conventions. The new leaders of Bangla Desh have been urging Bengalis not to take the law into their own hands.

However, Bangla Desh Premier Tajuddin Ahmed had pointed to a vested interest in the protection of Pakistanis. The idea was that the large numbers of Pakistanis in the east could be exchanged for the imprisoned Bengali hero Sheikh Mujibur Rahman and some 20,000 Bengalis who have been living in West Pakistan. This of course would be a practical solution to an extremely complex problem - and offers perhaps the only hope of saving the Sheikh's life (Review 50). But Islamabad's thinking on this is unknown.

General Niazi's surrender signals the birth of Bangla Desh as a fait accompli. It also means the birth of a new Pakistan, reduced in size and confined to India's western frontier. The military debacle in the east must surely have dealt a heavy blow to the hardline generals in Islamabad whose hawkishness started it all. The new Pakistan can only gain from what appears to be an inevitable leadership change - even if this means the rise to the top of the ambitious Zulfikar Ali Bhutto.