On his way towards Gora Kalwaria, Boncza-Uzdowski entered the territory controlled by the Wehrmacht (in the night of September 8/9, near Mszczonow). After a two-day march, he crossed the Vistula by a boat, and headed towards Warsaw, reporting to general Rommel. On September 13, he again took command of the 28th I.D., fighting in the Battle of Modlin. Captured by the Germans on September 29, Boncza-Uzdowski spent the war at Oflag VII-A Murnau.