Mr. Berg, second from right with beret, with other members of the Abraham Lincoln Brigade and Spanish drivers, kneeling, in Spain in 1938. (Abraham Lincoln Brigade Archives)

Delmer Berg, who left a dishwashing job in California to join the Republican forces fighting Gen. Francisco Franco during the Spanish Civil War of the 1930s, a bloody and ultimately futile struggle of which Mr. Berg was the last known American veteran, died Feb. 28. He was 100.

Thomas Berg said that his father died at his home in Columbia, Calif., and that the cause was complications from a fall.

His death was announced by the Abraham Lincoln Brigade Archives, an organization that honors the 2,800 American volunteers who, despite official U.S. neutrality in the conflict, fought to defend Spain’s elected government against Franco’s fascist insurrection.

The Spanish Civil War began in 1936 and pitted Franco’s Nationalists, backed by Nazi Germany and Fascist Italy, against the Republicans, or Loyalists, supported by the Soviet Union, in a conflict that presaged World War II.

Also supporting the Republicans were 40,000 members of the International Brigades. The Americans among them, a group that included Mr. Berg, made up the unit named in honor of the 16th president.

Mr. Berg at his home in Columbia, Calif., in 2015. (Andy Alfaro/Modesto Bee via AP)

Franco defeated his outgunned opponents in 1939 and established a dictatorship that would last until his death in 1975 — three decades beyond the defeat of Hitler and Mussolini. Hundreds of thousands of Spaniards, and a third of the Americans who participated in the conflict, lost their lives.

Like many of his compatriots, Mr. Berg was a communist and lifelong defender of progressive causes, including civil rights and organized labor. During the Depression, he left high school to help provide for his family as a farmworker, an experience that contributed to his sense of solidarity with the downtrodden. He once said that he “became a radical early on in life.”

He was employed at a hotel in Los Angeles when he noticed a billboard calling for volunteers to support the anti-fascist cause. He expressed interest and helped collect clothing for the Spanish before being selected to go to the front.

Mr. Berg took a bus to New York and then sailed to France, traversing the Pyrenees to enter Spain in early 1938. He served in field artillery and antiaircraft units, according to the brigade archives, and was credited with laying communications lines during the Battle of the Ebro River, one of the costliest engagements in the war.

In Valencia, he was wounded when a Fascist airplane mistakenly bombed the monastery where his unit was living, instead of its target, a railway station.

“I didn’t know I was hurt,” he told the New York Times last year. “There’s a lot of confusion with a bomb explosion; you’re kind of disoriented. We all started climbing down a pipe that led from the dormitory to the ground floor. I was the last one, and as I hung on that pipe, it put pressure on my chest. When I got down to the ground, I noticed that I had blood all over my front.”

Shrapnel had lodged permanently in his liver.

Delmer Esley Berg, a son of farmworkers, was born in Anaheim, Calif., on Dec. 20, 1915. He studied Latin in high school, an experience that would help him pick up Spanish later in life. He developed an affinity for Spanish culture by reading the Cervantes masterpiece “Don Quixote,” he told an interviewer with the Anderson Valley Advertiser in California.

Mr. Berg served briefly in the Army before obtaining a discharge and going to Spain, according to a son. He rejoined the military to serve in the Pacific during World War II, later returning to social activism.

In California, he supported himself by pruning trees, working in canneries and doing landscaping while organizing with the United Farm Workers. According to the brigade archive, Mr. Berg was “harassed” by the FBI during the anti-communist inquisitions of Sen. Joseph R. McCarthy (R-Wis.) in the 1950s.

Mr. Berg held a leadership position with the local chapter of the NAACP, the Modesto Bee reported, and participated in efforts to oppose nuclear proliferation and U.S. involvement in Central America.

“I felt so strongly about the Spanish people’s struggle that when I got back to the United States, I wanted to do the same thing here, in my own way,” he told the Times. “I wanted to remain active in the working people’s movement.”

Mr. Berg was married several times. His wife of 43 years, the former June Wilson, died in 2015. Survivors include two children from previous relationships, Thomas Berg of Cottage Grove, Ore., and Ernst Berg of Turlock, Calif.; two stepchildren, Michael Laughlin of Antioch, Calif., and Kathleen Wheat of Tuolumne County, Calif.; and numerous grandchildren and great-grandchildren.

Decades after the Spanish Civil War, the volunteers who fought alongside the Republicans retained a certain romantic appeal, fueled in part by their depiction in works such as Ernest Hemingway’s novel “For Whom the Bell Tolls.”

“As the last living veteran of the Abraham Lincoln Brigade, I feel a little isolated, but I cannot worry about that,” Mr. Berg told the Times last year. “I get a lot of letters from all over the country. Younger people write me — they want to know what happened. Can you tell me, they ask? You were there. All the rest of them are dead now.”