The first phase of the battle started on August 12, with a Red Army frontal assault on the Praga bridgehead. In heavy fighting, Radzymin changed hands several times and most foreign diplomats left Warsaw; only the British and Vatican ambassadors chose to remain. On August 14, Radzymin fell to the Red Army, and the lines of Władysław Sikorski's Polish 5th Army were broken. The 5th Army had to fight three Soviet armies at once: the 3rd, 4th, and 15th. The Modlin sector was reinforced with reserves (the Siberian Brigade, and General Franciszek Krajowski's fresh 18th Infantry Division—both elite, battle-tested units), and the 5th Army held out until dawn.