Armed Confederate veterans in New Orleans riot against a meeting of Radical Republicans, killing 48 people and injuring another 100.

The New Orleans massacre of 1866 occurred on July 30, when a peaceful demonstration of mostly Black Freedmen was set upon by a mob of white rioters, many of whom had been soldiers of the recently defeated Confederate States of America, leading to a full-scale massacre. The violence erupted outside the Mechanics Institute, site of a reconvened Louisiana Constitutional Convention. According to the official report, a total of 38 were killed and 146 wounded, of whom 34 dead and 119 wounded were Black Freedmen. Unofficial estimates were higher. Gilles Vandal estimated 40 to 50 Black Americans were killed and more than 150 Black Americans wounded. Others have claimed nearly 200 were killed. In addition, three white convention attendees were killed, as was one white protester.

The Radical Republicans were a political faction within the Republican Party originating from the party's founding in 1854—some six years before the Civil War—until the Compromise of 1877, which effectively ended Reconstruction. They called themselves "Radicals" because of their goal of immediate, complete, and permanent eradication of slavery in the United States. However, the Radical faction also included strong currents of nativism, anti-Catholicism, and support for the prohibition of alcoholic beverages. These policy goals and the rhetoric in their favor often made it extremely difficult for the Republican Party as a whole to avoid alienating large numbers of American voters of Irish Catholic, German, and other White ethnic backgrounds. In fact, even German-American Freethinkers and Forty-Eighters who, like Hermann Raster, otherwise sympathized with the Radical Republicans' aims, fought them tooth and nail over prohibition. They later became known as "Stalwarts".