Reuben Ainsztein was born to a Jewish family in Vilnius, Lithuania, then part of the Russian Empire, and grew up in Poland. Facing brutal pogroms and anti-Semitism, in 1936 he moved to Brussels, Belgium and enrolled in medical school. After the outbreak of World War II, he fled across France and Spain, where he was interned for 14 months, before finally making his way to the UK. He volunteered for the Royal Air Force (RAF) and served as a turret gunner on more than 20 bombing raids. During one mission, he had to parachute out but returned safely to Britain to convalesce. After the war, he learned that his parents and sister had all been murdered by the Nazis. Ainsztein, who spoke 11 languages, including Yiddish, Polish, Russian, and Hebrew, became a journalist, working for Reuters, the BBC, and The Sunday Times. He also contributed articles to The Times, The New Statesman, Jewish Quarterly, and other publications.
He was the author of two massive, critically acclaimed books: Jewish Resistance in Nazi-Occupied Eastern Europe (1993) and The Warsaw Ghetto Revolt (1992). His memoir In Lands Not My Own: A Wartime Journey, written in the 1940s or 1950s, was published posthumously in 2002.