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On the morning of Sept. 11, a small group of dedicated men launched a surprise attack that resulted in the vicious slaughter of men, women and children. The assailants were forced to act, they believed, because their religious community was under siege by the United States government.

They perpetrated their murders with the blessing of their leaders and in the name of God, although the religion's highest authorities would immediately disavow themselves of any connection with the massacre. In its wake, most Americans claimed the faith itself was inherently violent and supported military action against the associates of those who perpetrated the massacre.

This scene played out nine years ago in New York City and Washington, D.C., with al-Qaida's terrorist attacks on the World Trade Center and the Pentagon. But it also describes another Sept. 11 morning — in 1857 — when Mormon settlers in southern Utah, along with some Paiutes they had recruited, mercilessly killed more than 100 California-bound emigrants.

If history doesn't exactly repeat itself, it certainly has a profound sense of irony in placing two of the most horrific acts of religious violence in American history on the same day, nearly a century and a half apart.

For decades afterward, the Mountain Meadows Massacre was a flashpoint for opponents of Mormonism. Works of popular fiction depicted Mormons as bloodthirsty zealots whose primary form of religious devotion was murdering "gentiles." Testimony in the 1879 Reynolds v. U.S. trial placed Mormons alongside other "thugs" who "commit murder with impunity, on the ground that it was sanctioned and enjoined by their system of religious belief."

Some Southern mobs cited vengeance for the fallen emigrants as their rationale for vigilante violence against Mormon missionaries who were not even born at the time of the massacre.

The raw wounds of mass violence do not heal easily. It took four decades after this incident for the rest of America to drop its suspicions and grievances enough to pursue the path of accommodation.

Mormons then became one of the great success stories of the 20th century, trailing perhaps only Catholics and Jews as once-reviled religious minorities who earned a modicum of acceptance and achievement in American society. Enough skepticism remains on each side to preclude a full embrace, but Mormonism and America have at least become skilled, if wary, dancing partners.

Sept. 11, 1857 and 2001 are not historically or morally equivalent — nearly 30 times more people died in the latter, which then led to two international wars. The two tragedies can, however, offer similar lessons.

No group is immune to extremist violence. Mass murderers have killed in the name of God, the state, freedom, family and virtually any other ideal or institution we admire. The challenge is not only to denounce and isolate the extremist fringe in every community, but to engage in robust civic education and moral formation that focuses on tolerance, pluralism and compassion. These values are both religious and secular, and represent the most effective way to defuse the hard edges of identity-based discrimination and violence.

In the wake of Sept. 11, 2001, Muslims in America and beyond have a responsibility to marginalize their own extremists and do all in their power to prevent terrorism from being committed in their, and their religion's, name. But the rest of America also has a duty to live up to the principles of tolerance, pluralism and religious freedom enshrined in the nation's founding. Hate speech, arson at mosque construction sites, and burning Qurans do not, under any definition, live up to these standards.

The tragedy of Sept. 11, 1857, fueled decades of discrimination and violence against Mormons. Nearly one decade after the unfathomable tragedy of Sept. 11, 2001, we all have the opportunity — and the responsibility — not to repeat the mistakes of the past.

Patrick Mason is a historian and research associate professor at the University of Notre Dame's Kroc Institute for International Peace Studies and is the author of the forthcoming book The Mormon Menace: Violence and Anti-Mormonism in the Postbellum South.