Gerhart Eisler (1897-1968) was a journalist and prominent communist activist in Austria, Germany, the United States, and the German Democratic Republic (East Germany). Born in Leipzig, Gerhard Eisler grew up in Vienna. He served in the First World War and returned from the Front radicalized. In 1918, with his sister, Ruth Fischer, and his brother, the leftist composer Hans Eisler, he was among the founders of the Austrian Communist Party. In 1920 or 1921 he moved to Germany and joined the German Communist Party (KPD). From 1929 to 1931 he was a liaison between the Communist International (Comintern) and the Communist Parties in China, and then from 1933 to 1935 its liason to the United States. The Comintern then sent Eisler to Spain during the Spanish Civil War to head a radio station broadcasting antifascist programming and publish a newspaper for volunteers serving in the International Brigades. From Spain he went to Paris and wrote articles for KPD publications to be distributed clandestinely in Nazi Germany. In 1939 he was interned by French authorities. After more than a year of imprisonment an offer of asylum to Spanish Civil War veterans by the Mexican government allowed him to be released and depart France. In 1941, Eisler and his companion, Brunhilde Rothstein (who later became his third wife), were on their way to Mexico when the ship on which they were traveling was torpedoed and British authorities interned them in Trinidad for some weeks. They were then allowed to continue to New York, but upon arrival were detained. Although granted transit privileges, they were not actually allowed to resume their trip, but were forced to stay on Ellis Island, as in the face of the advancing World War, the United States government had issued an order forbidding Germans or Austrians transit or exit visas to Latin American countries. After a determined campaign of several months by their friends, they were granted permission to enter the United States.

In the U.S. the Eislers lived in New York City and Gerhart Eisler worked as a journalist and helped found the anti-Nazi newspaper,The German-American. In 1946, as Eisler prepared to return to Germany, he was publicly accused of being an agent of the Soviet Union and his permission to leave the United States was rescinded. He and his wife and friends were subjected to intense surveillance by the U.S. Federal Bureau of Investigation and other federal agencies, and in February 1947 he was arrested for misrepresenting his Communist Party affiliation on his immigration application. Several days later he was summoned to testify at a hearing of the U.S. House Un-American Activities Committee (HUAC). There he refused to be sworn in without first reading a statement and was charged with contempt of Congress. One of the witnesses at the hearing testifying against him was his sister, Ruth Fischer (the two had been estranged since 1933), who had become fiercely anti-Communist. Eisler was tried in two separate trials for both charges, and lost both. He was sentenced to one to three years in prison, but released on bond, while his cases were on appeal (the Supreme Court had agreed to consider his contempt charge). Shortly thereafter he was arrested by the U.S. Immigration and Naturalization Service in connection with deportation proceedings and held on Ellis Island for three months and was only released when he went on a hunger strike, putting him in the position of having the U.S. government attempting both to deport and detain him simultaneously. Eisler's case became a celebrated one, for both the American right (an American Cold War film, I Was a Communist for the FBI, featured an actor portraying Eisler as a leading character, a villainous foreigner secretly heading the American Communist Party) as well as the left.

In May 1949, Eisler escaped by stowing away on a Polish liner bound for London. Once in England, although initially detained by the authorities, the British government allowed him to leave a month later. Eisler flew to the German Democratic Republic, where he joined his brother, Hans (who had been deported from the U.S. a year earlier also after refusing to cooperate with HUAC). Eisler remained in East Germany the rest of his life, continuing to work in radio and as a journalist.