SIOUX CITY, Iowa — Anthony Pretrick was scraping by as a fisherman in his Pacific island homeland of Micronesia when he met a job recruiter with an irresistible offer: There was a fortune to be made slaughtering hogs in a faraway place called Iowa.

A new pork processing plant was hiring workers for $15.95 an hour — nearly 10 times what Mr. Pretrick, 26, made. He would get a one-way plane ticket. A hotel room. Free meals. Cash to send home to his wife and two sons. Hundreds of other Micronesians had already signed up.

“It was big money for us,” Mr. Pretrick said. “We left.”

Poverty propelled the Micronesians to take jobs trimming pork carcasses, jobs that few Americans want in an era of record low unemployment. And a quirk of law made Micronesians like Mr. Pretrick uniquely valuable to employers who might be worried about becoming the next target of the Trump administration’s workplace immigration raids: They were legal, allowed to work in the United States without visas or green cards under decades-old agreements rooted in America’s atomic testing and military history in the Pacific.

But what unfolded after 200 Micronesians made the 7,000-mile journey over the past year to the cornfields and hog farms of western Iowa became a tangled migration saga of what the workers called mistreatment and broken promises.