By 17th September, 1939, the Polish army had been completely broken. The Germans had reached the Vistula River and encircled Warsaw. On the same day the Russian armies crossed the eastern frontier of Poland and began their advance up to the Demarcation Line which had been agreed in the secret National Socialist-Soviet Pact of 23rd August. Next day they were in Vilna and Brest-Litovsk. Since there was not any logic, not any morality whatsoever attached to the British declaration of war against German, it did not suit the British to therefore declare war against Russia also for the same reason -- invasion of Poland. By 23rd September only the towns of Warsaw and Modlin continued to resist the German advance.