In 1984 the prime minister of India, Indira Gandhi, allowed government officers to storm one of Sikhism’s holiest temples in a search for separatists who were agitating for a new country. In response, her bodyguards assassinated her. That led to a multiple-day attack across India by the authorities and citizens against Sikhs. When it was over, the government said 3,300 Sikhs had died. Estimates by independent observers put the toll between 8,000 and 17,000. Other estimates put the number at about 30,000 people who lost their lives in the ethnic killing. No matter the number, Sikhs worldwide have considered the...