Kukliński was born in Warsaw to a working-class family with strong Catholic and socialist traditions. During World War II, his father became a member of the Polish resistance movement; he was captured by the Gestapo, and subsequently died in the Sachsenhausen concentration camp. After the war, Kukliński began a successful career in the Polish People's Army. In 1968, he took part in preparations for the Warsaw Pact's invasion of Czechoslovakia. Disturbed by the invasion, and by the brutal crushing of the parallel Polish 1970 protests, in 1972, Kukliński sent a letter to the US embassy in Bonn describing himself as an army officer from a Communist country, and requested a secret meeting.