“It’s my God-given right to be here,” said one balaclava-clad militia member who gave his name only as Viper. Chafing at the hostile reactions to the militia’s actions, he said that he was an Army veteran and that he expected his group, if pushed out, to set up camp in another location along the border.

“The guys in Washington say one thing about not wanting us on the ground but no one from the Border Patrol here has ever told me they don’t want our help,” he said, squinting under the midday sun. “We’re here to protect Americans from the illegals violating our sovereignty.”

The militia’s encampment on Tuesday was little more than a trailer and a few pickup trucks next to a newly installed “No Trespassing” sign. It appeared to reflect the impasse these armed vigilantes now find themselves in: under the magnifying glass of the F.B.I., cut off from funding, defending their actions to the public and torn asunder by the arrest of their leader, a resident of northwest New Mexico and a three-time felon who went by the alias Johnny Horton Jr. but whose real name is Larry Hopkins.