During the next days, army units retreating in face of the onslaughts of the German armor and under continuous air attacks, fought their way through the German lines to the besieged capital, in particular from the Battle of Bzura. After 13 September Warsaw and Modlin were effectively surrounded. The only way the Polish units were able to reach the besieged area was through the Kampinos Forest between Warsaw and the Vistula river. On 22 September German forces cut through the remaining lines of communication between Warsaw and Modlin.