As assistant attorney general heading the Antitrust Division of the U.S. Department of Justice, Thurman Wesley Arnold (1891-1969) spearheaded the campaign against corporate monopoly carried on by the Roosevelt administration.

Born on June 2, 1891, the son of a prosperous lawyer and rancher, Thurman Arnold grew up in and around Laramie, Wyo., which still retained much of its raw frontier character. At the age of 16, having graduated from the University of Wyoming Preparatory School, Arnold went east to enter Princeton University. Although his years at Princeton were, by his own account, ordinary, he was elected to Phi Beta Kappa, and he determined on a legal career. He received his law degree from Harvard in 1914 and established his practice in Chicago.

In the spring of 1916 Arnold's field artillery unit of the Illinois National Guard was ordered to Texas to assist the United States expedition into Mexico to search for the...