Mike Arnold had been trying to step back from a life in politics when the first alerts lit up his phone. As the volunteer mayor of tiny Blanco, Texas, he’d been vilified for turning the holiday parade back into a Christmas parade and harassed by his fiercest critics while filling potholes. He was mocked online until a few threats began to arrive in his mailbox. Politics had become blood sport. He finished his term in May and disappeared into his family’s construction business, before his phone drew him back.

“Watch this! We’re at war,” a friend wrote last month, and Arnold, 55, clicked on the link. He saw Charlie Kirk, a fellow Christian conservative, speaking to a crowd of students in Utah. He heard the echo of a rifle. He watched Kirk go down. Arnold had hunted enough deer to recognize a kill shot, so he began to pray not for Kirk’s survival but for the country he was leaving behind.

As mayor, Arnold had often warned his constituents about the decline of Christian values, the spread of indecency and the fragile “veneer of civility” holding America together. Now he believed it was fracturing. Retreat no longer seemed like an option. He went on Facebook, changed his profile picture to one of Kirk and joined the fight online.

“Time to take the gloves off,” he wrote. “Enough is enough.”

In the weeks since Kirk’s murder, millions of Americans have engaged in an internet war, flooding one another’s feeds with accusations and attacks that begin onscreen and spill into the real world in places like Blanco, a town of 2,100 in Texas Hill Country. Hundreds of people have been doxxed, fired or threatened for social media posts that were perceived as callous or celebratory in the wake of Kirk’s death. A historic act of political violence has unleashed a wave of new threats, deepening the cycle of division in a nation splitting into two hostile sides.

Arnold had always conceived of himself as a peacemaker, a bridge-building conservative who was opposed to Donald J. Trump in 2016 because of the way he stoked anger and hatred in campaign speeches that “crossed the line in terms of decency,” Arnold said. He wore a necklace made up of foreign coins from his mission trips to Nigeria, where he’d helped raise money to build a school for hundreds of children living in a camp for displaced people. He’d spent his professional career in ministry, sinking into personal debt to start a nonprofit that took thousands of children on their first hunting and fishing trips. Each summer, he taught teenagers how to use a rifle and then baptized them in rivers or horse troughs.