Pakistan carried on for 23 years like that, with the more numerous Bengalis in the east feeling increasingly neglected by their Punjabi brethren in the west, where the capital was. Things came to a head in December 1970, when Sheik Mujib-ur-­Rahman, a pipe-smoking Bengali leader, and his party, the Awami League, won the elections on the promise of autonomy for East Pakistan. (Whatever he wanted privately, he did not call for independence.) Rahman never got a chance to form a government. Gen. Agha Muhammad Yahya Khan, egged on by Zulfiqar Ali Bhutto, the second-place finisher, arrested Rahman and ordered the army to crush the Bengalis. Dominated by Punjabis, the army moved brutally, shooting and detaining Bengali leaders, intellectuals and anyone who opposed them.

Enter the United States. At the time of the elections, Pakistan, though ruled by a military dictator, was an American ally with an American-equipped military; India, the giant democracy, considered itself nonaligned — a neutral player in the Soviet-American standoff. Given what was happening on the ground — the Pakistani Army acting wantonly, ignoring the results of an election — you might expect the White House to restrain the Pakistani generals. So one arrives at the devastating heart of Bass’s book. (Note: I have interviewed Bass and met him socially a couple of times.)

At the time of the crackdown in East Pakistan, President Nixon and his national security adviser, Henry Kissinger, were trying to establish relations with the People’s Republic of China, which was only then emerging from the chaos of the Cultural Revolution. Nixon wanted desperately to extract the United States from Vietnam in something less than a catastrophic way and, as focused as ever on the Soviet Union, he and Kissinger believed that opening a channel to China could help them with the war while, at the same time, delivering a blow to the Soviets by exploiting their rivalry with the Chinese. Pakistan and, in particular, Yahya, its military leader, became Nixon’s secret liaison with the Chinese leader Zhou Enlai. Yahya helped lay the groundwork for the visits to China by Kissinger and then Nixon. It’s hard to overstate just how earth-changing Nixon and Kissinger regarded their trips to China — and how important they thought they were for bringing them about.

In practice, this meant that Yahya — a vain, shallow mediocrity — was suddenly considered indispensable, free to do whatever he wished in East Pakistan. With the White House averting its eyes, the largely Muslim Pakistani Army killed at least 300,000 Bengalis, most of them Hindus, and forced 10 million to flee to India. Bass lays out his indictment of the White House: Nixon and Kissinger spurned the cables, written by their own diplomats in Dacca (the capital of East Pakistan), that said West Pakistan was guilty of carrying out widespread massacres. Archer Blood, the counsel general in Dacca, sent an angry cable that detailed the atrocities and used the word “genocide.” The men in the White House, however, not only refused to condemn Yahya — in public or private — but they also declined to withhold American arms, ammunition and spare parts that kept Pakistan’s military machine humming. Indeed, Nixon regarded the dictator with genuine affection. “I understand the anguish you must have felt in making the difficult decisions you have faced,” he told Yahya.

The voices of Kissinger and Nixon are the book’s most shocking aspects. Bass has unearthed a series of conversations, most of them from the White House’s secret tapes, that reveal Nixon and Kissinger as breathtakingly vulgar and hateful, especially in their attitudes toward the Indians, whom they regarded as repulsive, shifty and, anyway, pro-Soviet — and especially in their opinion of Indira Gandhi. “The old bitch,” Nixon called her. “I don’t know why the hell anybody would reproduce in that damn country but they do,” he said.