The Invasion of Poland by Nazi Germany represented the beginning of World War 2, being a short and dynamic campaign, in which the Wehrmacht applied the concept of blitzkrieg in an early form.
Blitzkrieg (meaning lightning war in German) was a new way to wage war, intended to achieve quick and decisive victories, avoiding a static confrontation of attrition as in World War 1. A concentration of tanks, accompanied by motorized or mechanized infantry, and backed by close air support, would attack certain points of front-line, thrusting in depth, encircling and then annihilating stunned enemy forces.
The nature of the German-Polish border was itself a vulnerability for Poland. East Prussia - a large German enclave - was in the north, Slovakia - an ally of Germany, also used as a base of attack- in the south, and Germany itself in the west.
By Adrien Coffinet [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/deed.en)], via Wikimedia Commons
The Germans had organized their forces in two major army groups, which were to advance towards the Polish capital, Warsaw:
The Invasion of Poland began on September 1, 1939. After a week, the German Army had already penetrated deep into the Polish territory, reaching close to Warsaw.
Bundesarchiv, Bild 146-1978-120-11 / Schwahn / [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/de/deed.en)], via Wikimedia Commons
The Luftwaffe (German Air Force) played a crucial role, having not just numerical superiority of about 6 to 1 but also technologically superior aircraft to those owned by the Polish Air Force. The Germans achieved the air supremacy naturally, virtually removing the Polish aircraft from the game after two weeks.
The Wehrmacht (Armed Forces of Germany) also had a numerical superiority in tanks of about 3 to 1. The German panzer divisions were the spearhead that pierced the Polish defense. The Poles did not have even one armored division, their tanks (of which some really good for 1939) being scattered in smaller units.
The Polish Army resisted heroically in many occasions, from the battles fought near borders, but it was forced to retreat in the face of German military superiority. The most significant Polish counteroffensive took place in mid-September in what has been called the Battle of Bzura.
The Polish armies retreating from the western Poland launched an offensive, promising at first, on the flank of German Army Group South, which had penetrated deeply into the center of the country, but eventually, the Poles suffered a decisive defeat.
On September 17, 1939, the Soviet Union - another heavyweight neighbor - attacked Poland following a previous agreement with Germany regarding the partition of Poland. The only remaining Polish forces in the east were battalions of border guards and therefore, except for some minor clashes, the Soviets occupied with ease the eastern part of the country.
On 29 September, Warsaw surrendered after fierce bombings by the Luftwaffe. A few days later, the last Polish points of resistance were wiped out. The Invasion of Poland led Hitler's Germany at war with France and Britain, anticipating a major conflict in Western Europe.