German aggression

Following a number of German-staged incidents (Operation Himmler), which gave German propaganda an excuse to claim that German forces were acting in self-defense, the first regular act of war took place on September 1, 1939, at 04:40 hours, when the German Air Force (Luftwaffe) attacked the Polish town of Wieluń, destroying 75% of the city and killing close to 1,200 people, most of them civilians. Five minutes later, at 04:45 hours, the old German battleship Schleswig-Holstein opened fire on the Polish enclave of Westerplatte at the Free City of Danzig on the Baltic Sea. At 08:00 hours, German troops, still without a formal declaration of war issued, attacked near the Polish town of Mokra. Later that day, the Germans opened fronts along Poland's western, southern and northern borders, while German aircraft began raids on Polish cities. Main routes of attack led eastwards from the German mainland through the western Polish border. A second route carried supporting attacks from East Prussia in the north, and a German and allied Slovak tertiary attack by units (Army "Bernolak") from the territory of German-allied Slovakia in the south. All three assaults converged on the Polish capital of Warsaw.

The Allied governments declared war on Germany on September 3; however, they failed to provide Poland with any meaningful support. The German-French border was calm, although the majority of German forces, including eighty-five percent of their armoured forces, were engaged in Poland. This marked the beginning of the Sitzkrieg, Phony War. Despite some Polish successes in minor border battles, German technical, operational and numerical superiority forced the Polish armies to withdraw from the borders towards Warsaw and Lwów. The Luftwaffe gained air superiority early in the campaign. By 3 September, when Kluge in the north had reached the Vistula (some 10 kilometres from the German border at that time) river and Küchler was approaching the Narew River, Reichenau's armour was already beyond the Warta river; two days later his left wing was well to the rear of Łódź and his right wing at the town of Kielce; and by 8 September one of his armoured corps was on the outskirts of Warsaw, having advanced 140 miles in the first week of war. Light divisions on Reichenau's right were on the Vistula between Warsaw and the town of Sandomierz by 9 September, while List, in the south, was on the river San above and below the town of Przemyśl. At the same time, Guderian led his 3rd Army tanks across the Narew, attacking the line of the Bug River already encircling Warsaw. All the German armies had made progress in fulfilling their parts of the Fall Weiss plan. The Polish armies were splitting up into uncoordinated fragments, some of which were retreating while others were delivering disjointed attacks on the nearest German columns.

Polish forces abandoned regions of Pomerania, Greater Poland and Silesia in the first week of the campaign, after a series of battles known as the battle of the border. Thus the Polish plan for border defence was proven a dismal failure. The german advance wasn't slowed down, at least not as a whole, and furthermore the units retreating eastwards towards new defensive positions were outpaced by the Germans. On 10 September, the Polish commander in chief, Marshal Edward Rydz-Śmigły, ordered a general retreat to the southeast, towards the so-called Romanian bridgehead.

Meanwhile, the Germans were tightening their net around encircled Polish forces west of the Vistula (in the Łódź area and, still farther west, around Poznań) but also penetrating deeply into eastern Poland. Warsaw, under heavy aerial bombardment from the first hours of the war, was first attacked on 9 September and was put under siege from September 13. Around that time, advanced German forces had also reached the city of Lwów, a major metropolis of eastern Poland. 1150 German aircraft bombed Warsaw on September 24.

The largest battle during this campaign (Battle of Bzura) took place near the Bzura river west of Warsaw from 9 September to 18 September, when Polish armies Poznań and Pomorze, retreating from the border area of the Polish Corridor, attacked the flank of the advancing German 8th army. This attempted Polish counterattack failed after initial success. Defeat in this battle effectively marked the end of Polish ability to take the initiative and counterattack on a large scale.

The Polish government (of president Ignacy Mościcki) and the high command (of Marshal Edward Rydz-Śmigły) had left Warsaw in the first days of the campaign and headed south-east, arriving in Brześć on 6 September. General Rydz-Śmigły ordered the Polish forces to retreat in the same direction, behind the Vistula and San rivers, beginning the preparations for the long defence of the Romanian bridgehead area.