When the Allies created Poland out of the wreckage of World War I, they decided to give the new nation access to the Baltic. So they gave Poland dominion over Danzig, albeit as a Free City, and West Prussia, which became know as the Polish Corridor. In doing so, they physically isolated East Prussia from the rest of Germany, put a large number of Germans under Polish rule, and lit the fuse for World War II.
By 1939, Adolf Hitler had absorbed the Rhineland, Austria, Czechoslovakia, and Memel. He had rearmed and expanded the German Armed Forces, the Army [Heere], the Navy [Kriegsmarine] and the newly formed Air Force [Luftwaffe], collectively known as the Wehrmacht. He also felt under the gun. Hitler turned 50 in 1939, and believed that, like his mother, he would die young. So his thoughts turned to Lebensraum, and the Soviet Union. But between him, and it, lay Poland, with which he had a non-aggression pact since 1934.
So Hitler approached Poland with an offer of a junior partnership in the Axis. The entrance fee was the Corridor, Danzig and free passage for German troops to the U.S.S.R. Poland could expect German benevolence and territorial compensation [akin to what Poland had received from the dismemberment of Czechoslovakia]. Poland, which had received a blank check from Britain and France AFTER Czechoslovakia, refused, relying on its western allies to dissuade the Germans from action [Poland also refused to allow Soviet troops, as part of an Allied collective onto their soil].
Then Hitler pulled the rabbit out of the hat. HE signed a non-aggression pact with the Soviet Union, divvying up [in the Secret Protocols] Poland, the Baltic states and eastern Europe with Stalin. Without fear of a major two front war, Hitler had the OKH and OKW plan the attack on Poland.
Germany started with several crucial advantages. The first was geographic. Germany surrounded Poland on three sides: from the North [East Prussia], from the West [the Reich], and from the South [Czechoslovakia]. Additionally, except for defensible rivers, Poland, except in the South was flat as a pool table. The second was technological. The Germans surpassed, in quantity and quality, the Poles in tanks, aircraft, AT guns, motorized transport and most modern weapons. The third advantage was doctrinal. the Germans had developed mechanized, all arms coordinated warfare that the Poles [and almost no one else but the pre-purge Red Army had not.
The Germans deployed in three Army Groups. AG 'B', comprising the 4th and 3rd Armies, was commanded by Gen. Fedor von Bock. 4th Army [Kluge] faced the Polish Corridor from the West. 3rd Army [Kuechler] faced south from East Prussia. The wild card was XIX Pz. Korps [Guderian]. Attached to Kluge, Guderian's mission was to cross the Corridor and move south from East Prussia EAST of Warsaw [actually south east]. After taking the Corridor, Kluge [and Kuechler were to drive south
AG 'A' [Gerd von Rundstedt] comprised the 8th Army [Blaskowitz], 10th Army [Reichnau], and 14th Army [Weichs]. It was charged with attacking towards Warsaw from the southwest, while Weichs drove further east.
In the west, AG 'C' [Leeb], comprising 44 divisions held the Siegfried Line and faced the French [and British]. For all extents and purposes, the German Army was massed against Poland.
The offensive opened on 1 SEP 1939. The Germans were 'blessed' with the driest September in memory. Poland's dirt roads offered no impediment to the Germans. Neither did the Polish defense plan. Instead of holding the bulk of their forces behind the Vistula and Narew rivers, the Poles attempted to hold their entire border by deploying forward. They even maintained hopes of invading Germany.
The German plan, with one exception, worked like clock work. 4th Pz. was at Warsaw by Sept. 8th. Guderian blew through the Corridor, and then slashed southeast to Brest-Litovsk. The Germans developed a concentric double envelopment of the Poles.
The only fly in the ointment occurred at the Bzura River, where the Poles counter-attacked in force. Blaskowitz bore the brunt of the attack, but German mobility and air power turned the tide. Rundstedt wheeled 10th army units back to the west, and the Luftwaffe pounded the Poles mercilessly. It was over in less than a week.
To add to the Poles' misery, the Soviet Union attacked from the east on September 17th. The government fled to Romania, and by the end of September it was all over. Poland was partitioned [Guderian was forced to give Brest-Litovsk to the Soviets- and to take it back from them in June, 1941], and ceased to exist. Hitler returned West Prussia to the Reich, and created a "Government General" under Hans Frank out of the rump of what had been Poland. Poland would not be truly free for the next 50 years.
And the west? The French 'invaded' some several miles of Germany, then withdrew. They had no stomach for it.
Lessons learned: The Germans went into Poland with Panzer and "Light" divisions [an amalgam of armor, cavalry and God knows what else ]. The proved unwieldy, and of dubious value. By Spring, 1940, in time for the campaign in the West, they had been converted to Panzer Divisions. The Germans improved their communications, their air-ground coordination, and their combined arms doctrine over the winter.
Lessons unlearned: Poland, like the West the following Spring, was not a good test case. It was too small. The inherent weakness in the German Army was that it was two armies, a highly mechanized 10% of panzer and motorized troops, and a 90% of horse drawn, foot marching infantry and artillery. During the Channel drive in 1940, the increasing gap between the Panzers would be noted with concern. But the problem was not fixed [nor could it be, considering the ability, or lack thereof] of German industry to produce more vehicles [or the fuel they needed] by the time the Germans invaded Russia. And it cost them dearly.
German artillery reserves, and Luftwaffe bomb reserves were severely depleted in Poland. Refilling those reserves required a major effort, and took industry away from other tasks.
And it all started on September 1st, 1939.
