Isaac Deutscher (Polish: Izaak Deutscher) was a Polish-born Jewish Marxist writer, historian, journalist, political activist, and biographer, best known for his authoritative biographies of Joseph Stalin and Leon Trotsky, as well as his commentary on Soviet affairs. Born on April 3, 1907, in Chrzanów near Kraków, in what was then part of Austria-Hungary (now Poland), Deutscher came from a strictly Orthodox Jewish family. He received a traditional religious education and initially identified as a Hebrew-speaking Zionist, translating modern Hebrew poetry into Polish during his youth. In 1926, he joined the outlawed Polish Communist Party, where he was active as a poet and literary journalist until his expulsion in 1932 for opposing Stalinism. Fleeing rising antisemitism and political persecution, he moved to the United Kingdom just before the outbreak of World War II in 1939, becoming a British citizen.

In Britain, Deutscher established himself as a prominent intellectual and commentator on Marxism and international politics. He worked as a foreign correspondent for newspapers like The Manchester Guardian and contributed to various socialist publications. His major works include the biography 'Stalin: A Political Biography' (1949) and the acclaimed three-volume 'The Prophet' series on Leon Trotsky (1954, 1959, 1963), which portrayed Trotsky as a revolutionary prophet and offered nuanced critiques of Soviet history. Deutscher was a lifelong internationalist and self-described 'non-Jewish Jew,' emphasizing his secular, cosmopolitan outlook. He also wrote extensively on topics like the Russian Revolution, de-Stalinization, and global socialism, influencing leftist thought during the Cold War era.

Deutscher's career was marked by his independent Marxist perspective, often critical of both Stalinist orthodoxy and Western capitalism. He faced controversies, including expulsion from the Communist Party and debates over his Trotskyist sympathies, but remained a respected figure in academic and journalistic circles. He died on August 19, 1967, in London, leaving a legacy as a pioneering historian of revolutionary movements.