Isaac Deutscher (3 April 1907 – 19 August 1967), British journalist, historian and political activist of Polish-Jewish birth, became well-known as the biographer of Leon Trotsky and Joseph Stalin and as a commentator on Soviet affairs.

Deutscher was born in Chrzanów, a village in the Galicia region of Poland, then part of the Austro-Hungarian Empire, into a family of religiously observant Jews. He studied with a Hasidic rebbe and was acclaimed as a prodigy in the study of the Torah and the Talmud. By the time of his bar mitzvah, however, he had lost his faith. He "tested God" by eating unkosher food at the grave of a tzadik (holy person) on Yom Kippur. When God took no action, he became an atheist.

Deutscher first attracted notice as a poet, when at 16 he began publishing poems in Polish literary periodicals. His verse, in Yiddish and Polish, concerned Jewish and Polish mysticism, history and mythology, and attempted to bridge the gulf between Polish and Yiddish culture. He also translated poetry from Hebrew, Latin, German, and Yiddish into Polish.

Deutscher studied literature, history, and philosophy at the Jagellonian University in Cracow. At 18 he left Cracow for Warsaw, where he studied philosophy, and economics and became a Marxist. In 1927 he joined the illegal Polish Communist Party and soon became the editor of the party's underground press. In 1931 he visited the Soviet Union, where Moscow University offered him a post as a professor of the history of socialism and of Marxist theory. He declined these offers and returned to underground work in Poland.

In 1933, however, Deutscher published an article called "The Danger of Barbarism over Europe", in which he urged the formation of a united socialist-Communist front against Nazism. This contradicted the then official Communist line, which saw the social democrats, or "social fascists" as the greatest enemies of the Communist Party. Deutscher was expelled from the party, officially for "exaggerating the danger of Nazism and was spreading panic in the Communist ranks." He became a Trotskyist, but broke with official Trotskyism in 1938, being opposed to Trotsky's decision to found a Fourth International. Later he joined the Polish Socialist Party .

In April 1939 Deutscher left Poland for London. This move saved his life and paved the way for his future career. He never returned to Poland and never saw any of his family again. In London he worked as a correspondent for a Polish-Jewish newspaper and for a while joined the Trotskyist Revolutionary Workers League . When Germany occupied Poland he taught himself English and began writing for English magazines. He was soon a regular correspondent for the leading weekly The Economist. In 1940 he joined the Polish Army in Scotland, but was promptly interned as a dangerous subversive. Released in 1942, he joined the staff of The Economist and became an expert on Soviet affairs and European politics. He also wrote for The Observer as a roving European correspondent.

Deutscher's first major work, Stalin, A Political Biography was published in 1949. This was a controversial work, its intent more polemical than academic. The Cold War was underway, Stalin was still alive and Deutscher was still a committed Trotskyist, but in the book Deutscher gave Stalin what he saw as his due for building a form of socialism in the Soviet Union, even if it was, in Deutscher's view, a perversion of the vision of Marx, Lenin and Trotsky.

The Stalin biography made Deutscher a leading authority on Soviet affairs and the Russian Revolution. He followed it up with his most ambitious work, a three-volume biography of Trotsky: The Prophet Armed (1954), The Prophet Unarmed (1959) and The Prophet Outcast (1963). These books were based on detailed research into the Trotsky Archives at Harvard University. Much of the material contained in the third volume was previously unknown, since Trotsky's widow, Natalya Sedov , gave him access to the closed section of the Archives. Deutscher planned to conclude his series with a study of Lenin, but this work remained incomplete at the time of his death.

The upsurge of left-wing sentiment that accompanied the Vietnam War (see The Sixties) made Deutscher a popular figure on university campuses in both Britain and the United States. His Trotskyism had by then mellowed into a form of Marxist humanism, although he never renounced Trotsky. In 1965 he took part in the first "Teach-In" on Vietnam at the University of California, Berkeley, where thousands of students listened to his indictment of the Cold War. He was G. M. Trevelyan Lecturer at Cambridge University for 1966-67, and also lectured at the State University of New York, New York University, Princeton, Harvard and Columbia. The G. M. Trevelyan Lectures, under the title The Unfinished Revolution, was published after his sudden and unexpected death in Rome in 1967.

Despite being an atheist and a life-long socialist, Deutscher emphasised the importance of his Jewish heritage. He coined the expression "non-Jewish Jew" to apply to himself and other Jewish humanists. Deutscher admired Elisha ben Abuyah , a Jewish heretic of the 2nd century AD. But he had little time for specifically Jewish politics. In Warsaw, he joined the Communist Party, not the Jewish Bund, whose "Yiddishist" views he opposed. His definition of his Jewishness was: "Religion? I am an atheist. Jewish nationalism? I am an internationalist. In neither sense am I therefore a Jew. I am, however, a Jew by force of my unconditional solidarity with the persecuted and exterminated. I am a Jew because I feel the pulse of Jewish history; because I should like to do all I can to assure the real, not spurious, security and self-respect of the Jews."

Before World War II Deutscher opposed Zionism as economically retrograde and harmful to the cause of international socialism, but in the aftermath of the Holocaust he regretted his pre-war views, and argued that the creation of Israel was a "historic necessity" to provide a home for the surviving Jews of Europe. In the 1960s he became more critical of Israel for its failure to recognise the dispossession of the Palestinians, and after the Six Day War of 1967 he demanded that Israel withdraw from the occupied territories. "This 'six day wonder'", he commented, "this latest, all-too-easy triumph of Israeli arms will be seen one day... to have been a disaster... for Israel itself."

Books by Isaac Deutscher

• Soviet Trade Unions (1950)
• Russia After Stalin (1953)
• Russia, What Next? (1953)
• The Prophet Armed: Trotsky, 1879-1921 (1954)
• Heretics and renegades: and other essays (1955)
• Russia in transition, and other essays (1957)
• The Prophet Unarmed : Trotsky, 1921-1929 (1959)
• Great contest: Russia and the West (1960)
• The Prophet Outcast: Trotsky, 1929-1940 (1963)
• Stalin : a Political Biography (1966)
• Isaac Deutscher on the Israeli-Arab War: an interview with the late Isaac Deutscher (1967)
• The Unfinished Revolution: Russia 1917-1967 (G. M. Trevelyan lectures) (1967)
• Non-Jewish Jew and other essays (Edited by Tamara Deutscher) (1968)
• An Open Letter to Wladyslaw Gomulka and the Central Committee of the Polish Workers Party (1968)
• Russia, China, and the West 1953-1966 (Edited by Fred Halliday) (1970)
• Marxism in our time (Edited by Tamara Deutscher) (1971)
• Marxism, Wars, and Revolutions : essays from four decades (Edited by Tamara Deutscher) (1984)