The 'Battle of Łódź' was fought between the German and Polish forces during the 'Weiss' (i) German invasion of Poland (6/8 September 1939).

The German invasion had seen foreseen by the Poles as early as the spring of 1939, when Poland refused to join the Axis alliance against the USSR. The Polish strategy in the anticipated German aggression was to withstand the initial German attack and trigger France and the UK to declare war on Germany, and, then to undertake a fighting withdrawal to the 'Romanian bridgehead' in the Polish frontier region with Romania. Generał dywizji Juliusz Rómmel was given command of the Armia 'Łódź', and in order to buy time for the completion of his army’s mobilisation he led three divisions in the direction of the border. The Polish general believed that only through mobility and continuous resistance could the German advance be slowed for long enough to make possible the complete mobilisation of his army, whose headquarters was located in the city of Łódź. One of the reasons for the late mobilisation of the Polish forces was French and British political pressure not to mobilise, and it was only on 29 August that the Poles resumed their mobilisation

Germany launched 'Weiss' (i) on 1 September. Given their complete military superiority in terms of numbers and equipment, and the strategic advantage of having the Poles surrounded on three sides (in the west from mainland Germany, in the north from East Prussia and in the south from the former Czechoslovakia), the Germans hoped for a swift relatively bloodless victory. Adolf Hitler believed the French and British political and military leaderships were incapable of declaring war in the event of a German invasion Poland, but was mistaken. On 3 September both France and the UK declared war on Germany, but both countries lacked any means of proving any meaningful support, and the only Allied attack was the French 'Saar Offensive', which did not result in even a diversion of more German troops to the Western Front.

After the border 'Battle of Mokra', the German advance gained momentum and easily defeated the rear units of the Armia 'Łódź', which was still in the process of mobilisation. Łódź had fallen, and the three divisions sent to the border were cut off and ceased to exist. This triggered a domino effect: because Łódź had fallen, the Polish armoured brigade and its supporting infantry had no option but to withdraw from Piotrkow Trybunalski. This exposed the flank of the Armia 'Kraków', and both it and the fully mechanised 10^th Cavalry Brigade had to move toward Lwów. This movement out of south-western Poland in turn forced a withdrawal from northern Poland and left units under the de facto command of Generał dywizji Tadeusz Kutrzeba to fight the 'Battle of the Bzura River' and thereby become to the west of the Vistula river. Even the units that did manage to withdraw successfully for the most part failed to reach either the 'Romanian Bridgehead' or the Hungarian border crossing because on 12 September Soviet troops took over that very bridgehead and cut off the routes of escape as they invaded from the east. Only 60,000 to 80,000 Polish soldiers escaped German, Soviet and Slovak capture and had to go into some form of hiding if they were later to continue the fight as resistance fighters.

The Germans advanced too fast for the formations and units of the Polish army to position themselves for any counterattack, or for other armies to encircle the Germans by driving their spearheads into a small and narrow corridor between Łódź and Warsaw. The only major Polish offensive action occurred during the 'Battle of Bzura River Marshes', also known as the 'Battle of Kutno'.

The whole of Poland had fallen completely under German control in the west, Soviet control in the east and, on a very considerably smaller scale, Slovakia in the south by 6 October. By the following year, the city of Łódź had been renamed Litzmannstadt and became an important industrial centre for the German war machine, munitions and uniforms being manufactured in the newly established Ghetto Litzmannstadt by Jewish slave labour. Jews from Poland, Germany, Benelux and Czechoslovakia as well as Roma people from Austria were brought to work there under appalling conditions. While most of the slave labourers were later taken for extermination in the German concentration camps, more than 70,000 survived until the summer of 1944. But the Soviet advance to the west came to a stop and in August 1944 those survivors were also killed by the Germans. Toward the end of the war Łódź was taken relatively unscathered by Soviet forces on 17 January 1945. Only 877 Jews survived to the moment of liberation, and after it many thousands of ethnic Poles were expelled from the city.