During the German invasion of Poland and the siege of Warsaw, Landau took part in civil defense. During the subsequent occupation, he continued his research. He participated in underground education programs and worked with the Polish Underground State, preparing reports on the status of the wartime Polish economy. He supported the wartime Polish Socialist Party – Freedom, Equality, Independence, helped design their early wartime manifesto on the economy, and in 1940–1942 edited their underground newspaper.
