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{{See also|Molotov-Ribbentrop pact|Gleiwitz Incident|World War II: 1939}}

{{See also|Molotov-Ribbentrop pact|Gleiwitz Incident|World War II: 1939}}

In the immediate run up to WWII, there were frequent reports of trespassing Polish troops. On August 31, 1939 German covert operatives staged a fake attack by Polish troops on a German radio station. WWII started on September 1, 1939, when German troops invaded Poland. Hitler justified this as a defensive act, pointing to the frequent border incidents, and said famously that from this moment on Germany would strike back. In his speech to the [[Reichstag]] at the Kroll Opera House Hitler expressed his desire to resolve for all time the border disputes between Poland and Germany.<ref>https://avalon.law.yale.edu/imt/judpolan.asp</ref>

In the immediate run up to WWII, there were frequent reports of trespassing Polish troops. On August 31, 1939 German covert operatives staged a fake attack by Polish troops on a German radio station. WWII started on September 1, 1939, when German troops invaded Poland. Hitler justified this as a defensive act, pointing to the frequent border incidents, and said famously that from this moment on Germany would strike back. In his speech to the [[Reichstag]] at the Kroll Opera House Hitler expressed his determination to resolve for all time the border disputes between Poland and Germany.<ref>https://avalon.law.yale.edu/imt/judpolan.asp</ref>

The major tactical innovation of the war was the use of combined arms warfare, typified by the German doctrine of [[blitzkrieg]]. In this style of warfare armor, infantry, artillery and air power (see [[Luftwaffe]]) all coordinate to achieve overwhelming superiority at a point on the enemy lines. Armor and fast-moving infantry units then exploit the gap and penetrate deep behind enemy lines. The objective is to cause a widespread collapse of the enemy's ability to fight. It was particularly effective during the early stages of the war, before the Allies developed effective countermeasures.

The major tactical innovation of the war was the use of combined arms warfare, typified by the German doctrine of [[blitzkrieg]]. In this style of warfare armor, infantry, artillery and air power (see [[Luftwaffe]]) all coordinate to achieve overwhelming superiority at a point on the enemy lines. Armor and fast-moving infantry units then exploit the gap and penetrate deep behind enemy lines. The objective is to cause a widespread collapse of the enemy's ability to fight. It was particularly effective during the early stages of the war, before the Allies developed effective countermeasures.