Following the Battle Hostile_encounter of Raszyn on April 19 , the Polish forces commanded Control by Prince Józef Poniatowski left Warsaw undefended and withdrew Escaping to several fortresses located nearby ( most notably to Modlin Fortress and Serock ) .

However , the following day the besiegers were assaulted Attack from the back by a Polish division-sized sortie from the Modlin Fortress under Gen. Michał Sokolnicki .