In 1956, the young sociologist Nathan Glazer was asked by the social-democratic New Leader magazine to undertake a study of the controversial trial and execution of Ethel and Julius Rosenberg, who had been convicted of “conspiracy to commit espionage” and sent to their deaths in June 1953. Mr. Glazer zeroed in on Joel Barr and Alfred Sarant, two friends of Julius Rosenberg who had been all but ignored by the press and by the couple’s defenders and critics alike. They appeared to have been part of Rosenberg’s spy network and to have disappeared into thin air in 1950.

Mr. Glazer concluded with these words: “The story that has not been told is of espionage more extensive than we know.” The key, he thought, lay in finding out the truth about Rosenberg’s two missing friends. “The defenders of the Rosenbergs,” he wrote, “have not brought them forward to plead the innocence of their friends – or to clear their own names of the strong suspicion of Soviet espionage that now is attached to them.”

It has taken almost half a century, but Steven Usdin, in “Engineering Communism” (Yale University Press, 329 pages, $40), has finally told the story of the two men recruited by Julius Rosenberg to be Soviet spies and how they evaded the FBI and escaped to carry on their work on behalf of the Soviet state. Barr and Sarant rose to the pinnacle of power in the Soviet establishment and managed the building of the postwar modern Soviet military machine and microelectronics industry.

Mr. Usdin’s greatest accomplishment is to clear up remaining gaps in the story of the Rosenberg espionage network. The Rosenbergs’ defenders have long claimed that whatever the couple did, it was for genuine anti-fascist motives and that they only were concerned with helping an American ally. Mr. Usdin puts that argument to rest. He emphasizes that Rosenberg was recruited as a Soviet spy before June 1941 – i.e., during the years of the Nazi-Soviet Pact – and his primary motive was that he saw himself “as a partisan fighting behind enemy lines … on behalf of Soviet Communism.” He and his recruits wanted to “do anything they could to help the Soviet cause – before, during, and after the war against Hitler.” Barr, Sarant, and Rosenberg were Soviet patriots above all else.

The book clearly details the importance of the military information the network stole for Stalin. They passed on the 12,000-page blueprints for the Lockheed P-80 Shooting Star, the first American jet produced in large quantities and the workhorse of the Air Force in the Korean War. The detailed knowledge helped the Soviets build the MIG-15, whose superiority shocked the U.S. military in Korea. Barr and Sarant gave the KGB information on every project they worked on, including airborne radars for nighttime navigation and bombing and other new radar technology. One of these, an exact copy of the American device, was used in both Korea and Vietnam to direct artillery fire against American planes. “Rosenberg’s band of amateur spies,” Mr. Usdin writes, “turned over detailed information on a wide range of technologies and weapon systems that hastened the Red Army’s march to Berlin, jump-started its postwar development of nuclear weapons and delivery systems, and later helped communist troops in North Korea fight the American military to a standoff.”

Tipped off by the arrest of Ethel Rosenberg’s brother, David Greenglass, Barr and Savant made their extraordinary escape with KGB help. Once in Russia, they assumed new identities: Barr became Joseph Berg and Sarant became Philip Staros. The two Americans set out to prove to the Soviets that microelectronics was the future, and that computers would be based on solid-state electronics, a concept at odds with existing Soviet science. With Khrushchev’s support, they built Zelenograd, a new city composed of research institutions and production plants that would develop integrated circuits. It became a state sponsored version of America’s Silicon Valley.

Barr and Sarant’s most lasting legacy, one that extends to our new century, was the Soviet Kilo class submarines, whose torpedoes were guided by the Uzel digital computer the two Americans developed. Both men received the Soviet military’s Order of the Submariner in 1973 after the first successful Uzel test. As the author notes, if countries like Iran send oil tankers to the bottom of the Persian Gulf, or if Chinese subs sink Taiwanese vessels, the torpedoes used would be from the Uzel fire-control systems built by Staros and Berg – Alfred Sarant and Joel Barr.

Based on new files and personal access to the late Joel Barr, who gave Mr. Usdin interviews and letters revealing his entire life story, Mr. Usdin has made an important contribution to the story of Soviet espionage and to Cold War history. Most unbelievable is what happened to Joel Barr after he surfaced in 1990. Returning to the United States, Barr worried that he would be met at the airport by the FBI and quickly prosecuted for his crimes. He did not have to worry. Barr applied for and received monthly Social Security payments for the work he had done when he was spying for the Soviets, along with full citizenship rights, including the right to vote. Such is how America treats its homegrown traitors.

Mr. Radosh, an adjunct senior fellow of the Hudson Institute, is co-author of “The Rosenberg File.”