By 5 September, XIX Corps had linked up with forces advancing west from East Prussia. Guderian had accomplished his first operational victory and he gave a tour of the battlefield to Hitler and Heinrich Himmler, head of the SS. The next day he shifted his corps across East Prussia to participate in the advance on Warsaw. On 9 September his corps was reinforced by 10 Panzer Division and he continued deeper into Poland, finishing at Brest-Litovsk. In ten days Guderian's XIX Corps advanced , at times against strong resistance. The tank had proved itself to be a powerful weapon, with only 8 destroyed out of 350 employed. On 16 September, Guderian launched an attack on Brest Litovsk; the next day the Soviet Union invaded Poland. He issued an ultimatum to the city – surrender to the Germans or Sovietsthe garrison capitulated to the Germans. The Soviet Union's entry into the war shattered Polish morale and Polish forces began to surrender en masse to Guderian's troops. At the conclusion of the campaign, Guderian was awarded a Knight's Cross of the Iron Cross.