Editing Heinz Guderian (section)

===Invasion of Poland===
{{Main|Invasion of Poland}}

[[File:Armia Czerwona, Wehrmacht 22.09.1939 wspólna parada.jpg|thumb|upright|Guderian with [[Mauritz von Wiktorin]] (left) and Soviet ''[[Kombrig]]'' [[Semyon Krivoshein]] at the [[German–Soviet military parade in Brest-Litovsk]] after the invasion of Poland, 1939]]
During August, 1939 Guderian took command of the newly formed [[XIX Army Corps]]. At short notice he was ordered to spearhead the northern element of the [[invasion of Poland]] which began on 1 September.{{sfn|Hart|2006|p=50}} Under his [[corps]] command was one of Germany's six panzer divisions; Guderian's corps controlled 14.5 per cent of Germany's [[armoured fighting vehicles]]. His task was to advance through the former [[West Prussian]] territory (which included his birthplace of Kulm), then travel through [[East Prussia]] before heading south towards [[Warsaw]].{{sfn|Hart|2006|p=51}} Guderian used the German concept of "leading forward", which required commanders to move to the battlefront and assess the situation. He made use of modern communication systems by travelling in a radio-equipped command vehicle with which he kept himself in contact with corps command.{{sfn|Battistelli|2011|p=17}}

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.{{sfn|Hart|2006|p=53}} 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 {{convert|330|km|mi}}, at times against strong resistance. The tank had proven itself to be a powerful weapon, with only 8 destroyed out of 350 employed.{{sfn|Battistelli|2011|p=18}} On 16 September, Guderian launched an [[Battle of Brześć Litewski|attack on Brest Litovsk]]; the next day the Soviet Union invaded Poland. He issued an ultimatum to the city—surrender to the Germans or Soviets{{snd}}the 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.{{sfn|Hart|2006|p=56}} At the conclusion of the campaign, Guderian was awarded a [[Knight's Cross of the Iron Cross]].{{sfn|Fellgiebel|2000|p=171}}

The historian Russell Hart writes that Guderian supported the invasion because he "despised the Catholic, Slavic Poles who now occupied parts of his native, beloved Prussia".{{sfn|Hart|2006|p=47}} Foremost in his mind was the "liberation" of his former family estate at Gross-Klonia; Guderian ordered the advance on Gross-Klonia at night and through fog, leading to what he subsequently admitted were "serious casualties".{{sfn|Hart|2006|p=52}}

During the invasion, the German military mistreated and killed prisoners of war, ignoring both the [[Geneva Convention on Prisoners of War (1929)|Geneva Convention]] and their own army regulations.{{sfn|Epstein|2015|p=126}} Guderian's corps withdrew before the [[SS]] began its [[Nazi crimes against the Polish nation|ethnic cleansing campaign]]. He learned of murder operations and of Jews being forced into [[Nazi ghettos]] from his son, [[Heinz Günther Guderian]], who had witnessed some of them. There is no record of him having made any protest.{{sfn|Hart|2006|p=58}}