The 'Battle of the Polish Border' comprised the many engagements between German and Polish forces in the first days of the German 'Weiss' (i) invasion of Poland 1/4 September 1939).

This series of tactical battles ended in a German operational and strategic victory as the Polish forces were either destroyed or forced into precipitate retreat.

The Polish 'Plan Zachód' defence scheme, for implementation in the event of hostilities with Germany, was posited on a defence of Poland’s western border in the regions close up against that frontier. Much of Poland’s new industrial areas and major population centres were located in the western border area, and especially in Silesia, but its length rendered the border very difficult to defend properly with Poland’s limited resources. The plan was criticised by some of the Polish military and Western advisers, but supported by politicians who feared the effect of abandoning a significant part of the population to the Germans without a fight, and who were further discouraged from abandoning those territories as Poland’s allies, France and the UK, had not guaranteed the borders of Poland and might well decide to allow the Germans to take the Polish corridor top the Baltic Sea that they demanded in exchange for peace.

The German 'Weiss' (i) invasion plan called for the start of hostilities before the declaration of war, and for an early iteration of the Blitzkrieg (lightning war) to be employed. As laid down in 'Weiss' (i), German formations and units were to invade Poland from three directions: from the German mainland through the Poland’s western border; from the north from the Germany’s detached East Prussia region' and from the territory of Slovakia, accompanied by allied Slovak units. All three of the assaults were to converge on Warsaw on the Vistula river, while the main strength of the Polish army was to be encircled and destroyed in the region to the west of the Vistula river.

With a smaller population and thus a more limited military budget and accordingly an army of less size than that of Germany, Poland was further disadvantaged by the facts that it was not sure if the war would start without any form of political warning, and that its army was therefore not fully mobilised by 1 September.

The 'Battle of the Polish Border', which was thus a campaign based on a number of engagements rather than a single battle, began at about 05.00 on 1 September as German troops started to surge across the Polish border in numerous places. The 'Battle of Westerplatte', which is often described as having begun as 04.45 with the salvoes of the pre-dreadnought battleship Schleswig-Holstein on Polish coastal fortifications, is commonly described as the first battle of the war. Others have described the 04.45 salvoes as happening minutes after Luftwaffe attacks on Polish airfields. Several historians have identified the first action of the war as the bombing of the key Tczew bridge in the 'Polish corridor' by dive-bombers at about 04.30. The 'Himmler' false-flag operation had begun some hours earlier.

At 08.00 on 1 September, and still without any formal declaration of war, German troops, attacked near the Polish town of Mokra and the 'Battle of the Polish Border' had begun. Later that day, the Germans opened fronts along Poland’s western, southern and northern borders, while German aircraft began to bomb Polish cities. The main axes of attack led to the east from Germany proper through the western Polish border. A second axis carried supporting attacks from East Prussia in the north, and there was a collaborative German and Slovak tertiary attack by formations from the territory of the German-allied Slovakia in the south.

In the north-west, Generaloberst Fedor von Bock’s Heeresgruppe 'Nord' attacked Pomerania and Greater Poland, moving from Germany proper (General Günther von Kluge’s 4^th Army) and from East Prussia (General Georg von Küchler’s 3^rd Army). In the 'Battle of the Tuchola Forest', which lasted from 1 to 5 September, the German forces split Generał dywizji Władysław Bortnowski’s Armia 'Pomorze', which was tasked with the defence of the 'Polish corridor'; parts of it under Admiral Józef Unrug continued to defend pockets of the coast over the next few days or weeks in the 'Battle of Westerplatte', the 'Battle of Gdynia', the 'Battle of Hel' and others, while the rest was forced, together with Generał dywizji Tadeusz Kutrzeba’s Armia 'Poznań', to retreat to the east from their defensive lines in Greater Poland in the direction of Kłodawa in Kujawy.

In the northern Polisah region of Masovia, by 3 September part of 3^rd Army had defeated Generał dywizji Emil Krukowicz-Przedrzymirski’s Armia 'Modlin' in the 'Battle of Mława'. Polish forces retreated toward their secondary lines of defence on the Vistula and Narew rivers, allowing the Germans to move toward Warsaw, their main objective.

In the south and south-west, Heeresgruppe 'Süd' struck along the line dividing Generał dywizji Juliusz Rómmel’s Armia 'Łódź' from the Armia 'Poznań' and Generał brygady Antoni Szylling’s Armia 'Kraków'. Despite several Polish tactical successes, as in the 'Battle of Mokra' on 1 September, the Polish forces were soon forced to retreat as the 'Armia 'Łódź' was being outflanked by General Johannes Blaskowitz’s 8^th Army and General Walter von Reichenau’s 10^th Army. The Armia 'Kraków' was retreating from Silesia, and in the south Generał dywizji Kazimierz Fabrycy’s Armia 'Karpaty' was being slowly pushed to the north toward the Dunajec and Nida rivers by General Wilhelm List’s 14^th Army.

By 6 September Polish forces were in full retreat and Marszałek Polski Edward Rydz-Śmigły, the Polish commander-in-chief, ordered all his forces to fall back to the secondary lines of defence on the Vistula and San rivers.

Nearly all the battles considered part of the 'Battle of the Polish Border', with the exception of the 'Battle of Hel', which lasted for more than a month and the 'Battle of Mokra', which was a Polish defensive victory, resulted in rapid defeat of Polish forces, which were forced to abandon the Pomerania, Greater Poland and Silesia. Those defeats in turn made it more difficult for the Polish forces to fall back in any organised fashion to the secondary lines of defence behind the Vistula river and near the Romanian 'bridgehead'.

The 'Battle of the Polish Border' included the following battles which ended before or on 3 September: the 'Defence of the Polish Post Office in Danzig' on 1 September, the 'Battle of Chojnice' on 1 September, the 'Skirmish of Krojanty' on 1 September, the 'Battle of Lasy Królewskie' on 1 September, the 'Battle of Mokra' on 1 September, the 'Defence of Katowice' on 3/4 September, the 'Battle of Pszczyna' on 1/4 September, the 'Battle of Grudziądz' on 1/3 September, the 'Battle of Mława and Ciechanow' on 1/4 September, the 'Battle of Jordanów' on 2 September, the 'Raid on Fraustadt' on 2 September, and the 'Battle of Węgierska Górka' on 1/3 September.

The battles which began before 3 September and ended before or on 7 September included the 'Battle of the Tuchola Forest' on 1/5 September, the 'Battle of Borowa Góra' on 2/5 September, and the 'Battle of the Westerplatte' on 1/7 September.

The battles which began after 3 September and ended before or on 7 September included the 'Battle of Różan' on 4/6 September, the 'Battle of Piotrków Trybunalski' on 5/6 September, and the 'Battle of Tomaszów Mazowiecki' on 6 September.

The battles which began before 7 September included the 'Battle of the Wizna' on 6/10 September and the 'Battle of Hel' on 1 September/2 October.