Germany and Poland were the main opposing forces in the Polish September Campaign. The September Campaign was the joint Invasion of Poland by Nazi Germany, the Free City of Danzig, the Soviet Union, and a small Slovak contingent marking the beginning of WW II in Europe. Tactics and military hardware used in the Invasion of Poland varied between the two sides; The German economy was geared toward military production which supplied their armed forces with equipment that was often superior to their Polish counterparts. Old tactics such as Vernichtungsgedanke and the concept of the Schwerpunkt combined with newly mechanized units punched holes in Polish lines and close air support provided by the world class Luftwaffe disrupted Polish supply and communications lines. Other German tactics included the targeting of civilian targets in terror bombings to inflict huge losses amongst the civilian population, sending streams of refugees out of afflicted areas, thereby hampering Polish logistics. In 1939 the German navy also had an advantage over the tiny Polish fleet. Preparations for a defensive war with Germany were ongoing for many years but most plans assumed fighting would not begin before 1942. The Polish Army had about a million soldiers, but fewer than half had been mobilised by 1 September due to political pressure from France and Britain. The Polish-Soviet War showed Poland the benefits of mobility in military conflicts but Polish officials were unwilling (and unable) to invest heavily to make that a reality. Polish Cavalry Brigades were still effectively used as mobile infantry but in the end fell against German motorized units, however Germany also used cavalry for transportation during the September Campaign. The Polish Air Force lacked modern fighter aircraft for its highly trained pilots, while the Luftwaffe was much more numerous and had superior aircraft. The Polish Navy was a small fleet composed of destroyers and submarines, some of which survived the campaign by escaping to the North Sea to join with the Royal Navy. Germany Germany was only moderately prepared for the invasion of Poland; stocks of ammunition and replacement vehicles, for example, were low, and historians regard Hitler's move as a military gamble as well as a political one. The German economy had for years geared toward production of military equipment and supplies, and was capable of creating an army that could successfully invade its neighbours only if the campaign was not a long one. In fact,