Like many Americans, I reacted to the murders at the Sikh temple in Oak Creek, Wisconsin, with horror, and to the apparent arson at a Joplin, Missouri, mosque with sadness.

But I did not react with shock.

As the adviser to the Sikh Association at Boston University and a professor of many Muslim students, I am aware of the day-to-day discrimination these religious minorities experience in the United States. And as a historian I am aware of the history of discrimination against both groups throughout U.S. history.

The first decade of the 20th century saw a series of riots against Sikh laborers. On September 4, 1907, in Bellingham, Washington, hundreds of men attacked "Ragheads," as they called them, burning their homes, beating them up, looting their property, and driving them across the border into Canada. Similar riots occurred in that decade elsewhere in Oregon, Washington, and California.

Immediately after 9/11, a Sikh named Balbir Singh Sodhi was killed at a gas station in Mesa, Arizona, by a bigot who mistook him for a Muslim.

We Americans flatter ourselves as citizens of a "land of liberty" where religious freedom is sacrosanct. And we have much to brag about in this regard, not least a First Amendment that guarantees religious liberty and the separation of church and state. Yet the United States also has a long history of religious bigotry.

In a 2007 speech on "Faith in America," Mitt Romney courageously outlined not only "our grand tradition of religious tolerance and liberty" but also key moments when we traded in that tradition for hate. He spoke of Ann Hutchinson, the Puritan renegade banished from colonial Massachusetts and of Brigham Young, who led Mormons persecuted in the East to freedom in the West.

All too often, Romney observed, "Americans were unable to accommodate their commitment to their own faith with an appreciation for the convictions of others to different faiths."

Sikhism is one of the world's leading religions, with 22 million followers in India and sizable Sikh populations in the United Kingdom, United States, and Canada. This 16th-century religious creation echoes both Hindu devotionalism and the Sufi mysticism of Islam. Like Hindus, Sikhs believe in the karmic cycle of life, death, and rebirth. Like Muslims, they are monotheists who are wary of picturing God in human form.

Yet Sikhism is a distinct religious tradition, with its own traditions, including the practice of the langar meal, open to all people regardless of race, ethnicity, caste, or religion.