He was one of nearly 3,000 American volunteers who joined the Abraham Lincoln Brigade in the Spanish Civil War, where he served as a political commissar.

[1] Stjepan Mesaroš (sometimes spelled Mesarosh) was born on 1 January 1903 in Subocka, Croatia when it was still part of the Austro-Hungarian Empire.

[4]In 1920, Mesaroš emigrated with his mother and three sisters to the United States, specifically to Philadelphia, Pennsylvania where his uncle lived.

[8] In the fall of 1923, by now using the Americanized name "Steve Nelson", he relocated to Pittsburgh, which he was told was "a good labor town" with better prospects of finding employment.

With the onset of the Great Depression in late 1929, he and his wife moved to Chicago and began working full-time as Communist Party functionaries.

Two weeks prior, the Chicago Red Squad unit arrested him, Joe Dallet, and other Party activists.

[12] With the outbreak of civil war in Spain, Nelson immediately tried to join the Abraham Lincoln Brigade of American volunteers, but he was told by the CPUSA District Committee that he was needed in Pennsylvania to organize anthracite coal miners.

[15] However, once the Republican side suffered a severe setback at the Battle of Jarama in February 1937, Nelson, Dallet and 23 others were allowed to fight in Spain.

[6] The latter conflict started badly for the Republican side—only two soldiers out of 22 survived the first attempt to take a Nationalist stronghold in the town's church.

Nelson then led his men in a successful diversionary attack, and Amlie's soldiers were able to enter the fortified town.

After convalescing in Valencia, Nelson was briefly given the task of escorting prominent visitors to the Spanish front, such as John Bernard, Dorothy Parker and Lillian Hellman.

[18] In November 1937, he was recalled home by CPUSA leader Earl Browder, who wanted Nelson to go on a speaking tour to educate Americans about the Spanish Civil War.

After several years on the West Coast, the Nelson family headed east when he was elected to the National Board of the Party.

[14] The FBI planted listening devices in Nelson's residence and discovered that Weinberg had supplied "highly secret information regarding experiments being conducted at the Radiation Laboratory, Berkeley, pertaining to the atomic bomb."

Nelson was running a secret "control commission" to find informants and spies in the California branch of the Communist Party.

[22] However, according to historian John Earl Haynes, the FBI subsequently concluded that the many Soviet attempts to obtain vital data about the Manhattan Project were ultimately not successful.

[25] Years afterward, Nelson admitted he did not fully grasp the Fifth Amendment because he thought he could selectively answer some questions and invoke the Fifth for others.

As he put it, the HUAC's chief examiner that day, Congressman Richard Nixon, was "threatening me with contempt before the hearings were five minutes old.

During this period, Nelson wrote books about his experiences in Spain (The Volunteers) and his sedition trial and imprisonment (The 13th Juror) to raise money to sustain him and his family.

[1] In the aftermath of Nikita Khrushchev's secret speech about atrocities committed under Joseph Stalin's rule, Nelson resigned from the CPUSA in 1957.