With the German invasion of Poland in September 1939, the 1st Infantry Division advanced toward Warsaw as a component of the XXVI Army Corps in von Küchler's 3rd Army. It engaged Polish forces near the heavily defended town of Mława (see Battle of Mława) for several days, then crossed over the Bug and Narew Rivers. It fought again near Węgrów and Garwolin and ended the campaign east of Warsaw.