Charleston. Chattanooga. Colorado Springs. In these towns and dozens of other communities around the nation, 2015 was a year marked by extraordinary violence from domestic extremists — a year of living dangerously.

Antigovernment militiamen, white supremacists, abortion foes, domestic Islamist radicals, neo-Nazis and lovers of the Confederate battle flag targeted police, government officials, black churchgoers, Muslims, Jews, schoolchildren, Marines, abortion providers, members of the Black Lives Matter protest movement, and even drug dealers.

They laid plans to attack courthouses, banks, festivals, funerals, schools, mosques, churches, synagogues, clinics, water treatment plants and power grids. They used firearms, bombs, C-4 plastic explosives, knives and grenades; one of them, a murderous Klansman, was convicted of trying to build a death ray.

The armed violence was accompanied by rabid and often racist denunciations of Muslims, LGBT activists and others — incendiary rhetoric led by a number of mainstream political figures and amplified […]