His last major work, "Historia Polski 1914 – 1939" ("History of Poland 1914-1939"), was printed posthumously. Due to the government crackdown on opposition the book was heavily censored, but the underground errata was soon realised in the form of "bibuła" (Russian "samizdat"). The censorship of his books is a good example of the problem faced by historians under regimes which interfere with academic work. Specifically, the Polish communist censorship completely removed any mention of the Molotov-Ribbentrop Pact (Nazi Germany-Soviet alliance) and the section on the Soviet invasion of Poland on 17 September 1939 together with the photo of the German–Soviet military parade in Brest-Litovsk was removed and replaced with a general note on changed borders.