Berg died Sunday at his home in Columbia, Marina Garde from the Abraham Lincoln Brigade Archives in New York confirmed in a written statement.
The Modesto Bee reports Tuesday that friend Pat Cervelli said Berg, who she knew for three decades, became politically active early in life.
Berg was among about 2,800 members of the Abraham Lincoln Brigade who sought to defend an elected government from a military rebellion led by Gen. Francisco Franco. The revolt was successful, and Franco led Spain for decades.
"I was a worker," Berg told the newspaper. "I was a farmer. I was in support of the Spanish working people, and I wanted to go to Spain to help them."
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He was wounded in a bombing at a monastery. Born in Anaheim on Dec. 20, 1915, Berg spent part of his childhood on a farm near Manteca. Later, Berg lived in Modesto and was the only white member of the local chapter of the National Association for the Advancement of Colored People.
Berg served two stints in the U.S. Military: once in the National Guard before going to Spain and again in the East Indies during World War II.
Berg later worked as a landscaper and a stonemason. He and his wife, June, who preceded him in death, even built a home of stone and reused lumber.