CAMPO – As the sun set, Roy Wood looked out of place patrolling a dirt road a few steps north of a rusty fence that separates the United States from Mexico. The clean-shaven English-as-a-second-language instructor wore a T-shirt tucked in clean blue jeans, a pistol strapped to a belt. Many of the hundreds who make up the self-appointed civilian patrols monitoring the border to deter smuggling of people and drugs are unemployed or underemployed ex-military men who have long resented Mexicans who come to the United States illegally and, in their view, compete for jobs, crowd hospitals and schools and...