Complex sentence: After the Polish defeat Rowecki managed to avoid capture and returned to Warsaw and in October 1939 he became one of the leaders, then in 1940 commander, of the Związek Walki Zbrojnej.
Split into simpler sentences: After the Polish defeat, Rowecki managed to avoid capture and returned to Warsaw. In October 1939, he became one of the leaders, then in 1940 commander, of the Związek Walki Zbrojnej.