Some of President Trump’s most ardent campaign supporters were among his most vocal opponents on Thursday after he ordered the missile strike against Syria, charging him with breaking his promise to keep the United States out of another conflict in the Middle East.

Prominent writers and bloggers on the far right attacked Mr. Trump. They accused him of turning against his voters by waging an attack that he had for years said would be a terrible idea. They also criticized him for launching the strike without first seeking congressional approval — something he said on Twitter in 2013 would be a “big mistake.”

The most vocal in their outrage were leaders from the small but influential white nationalist movement.

Paul Joseph Watson, an editor at the conspiracy theorist site Infowars, said on Twitter that Trump “was just another deep state/neocon puppet.” He added, “I’m officially OFF the Trump train.”