On September 1, 1939, at dawn the German Third Reich invaded the Polish Second Republic along the full length of the border between the two countries. This action was carried out according to the command of Chancellor Adolf Hitler, was not accompanied by a declaration of war, and broke a non-aggression pact. It constituted the outbreak of the Second World War and resulted in the gradual occupation of Polish territories. Initial bombs were dropped on Wieluń and on a Polish outpost, run by Major Henryk Sucharski, on Westerplatte.
Battles on the frontier revealed that Hitler's German forces were much stronger than the Polish forces. The Wehrmacht very quickly gained control over the western part of Poland and headed in the direction of Warsaw.
The Second Republic's allies - Great Britain and France - entered the war on September 3, 1939, but limitations on their military activity left the Poles' hopes unfulfilled.
Meanwhile, the German army was held back - during the Battle of the Bzura on September 9-18, 1939 - by the joint offensive of General Tadeusz Kutrzeba's "Poznań" army and General Władysław Bortnowski's "Pomorze" army. This prolonged the struggle, resulting in great losses for both sides.
From the beginning of the war, Warsaw suffered aerial attacks, and on September 8, 1939, German forces reached the outskirts of the capital. Military actions destroyed cultural and historical monuments, the city's infrastructure, and 10% of residential buildings. Libraries, archives and artistic works were also ruined.
Initially, a few units of the Polish Army fought in defense of the city. Starting on September 3, 1939, General Walerian Czuma commanded them. He gradually brought order to numerous military units that had been retreating separately in the direction of Poland's capital. In this way, on September 8, 1939, he organized the "Warsaw" army, which was placed under the command of General Juliusz Rómmel.
Alongside the soldiers, a great number of Warsaw's civilians actively participated in the resistance. They were involved in the digging of antitank trenches, in the creation of barricades and fortifications, as well as being organized into various bodies: Anti-aircraft Defense, Civic Services and the Fire Department. Sapper units played an important role: on September 8-10, 1939, they prepared anti-tank barricades and land mines on the outskirts of Warsaw, making access to Warsaw impossible for German forces.
From the beginning of the conflict, the civil authorities - with the Civil Commissioner and Mayor Stefan Starzyński working alongside the commanding forces of the Battle of Warsaw - played a crucial role. Despite difficulties, they organized aid for the city's inhabitants and kept spirits up.
Refugees from Poland's western regions, having been forced from their homes, began arriving in the city. There were around 300,000 of them. They endured under extremely difficult conditions, with regard to both shelter and food.
On September 17, 1939, without declaring war, the Soviet army marched onto the eastern territories of the Second Polish Republic, in accordance with the Molotov-Ribbentrop Pact of August 23, 1939. Upon hearing about this, the Polish government, part of the army, and the police crossed the border into Romania and were interned there. On occupied territories, the Soviet forces initiated repressive measures against the Polish population (in accordance with the commanding forces of the USSR, under the leadership of Joseph Stalin, Polish Army officers, State Police officers, Border Control officers, as well as government workers, many of whom were interned in 1939, were massacred in April 1940 in Katyn, Miednoye, Starobelsk, and Ostashkov. Polish intelligentsia and landowners, who were considered political enemies, were sent by the People's Commissariat for Internal Affairs (NKVD) to forced labor camps on Soviet territory.)
The occupation of The Borderlands (Kresy) by the Soviet army, hastened the occupation of Poland by the German forces. On September 27, 1939, Warsaw surrendered (6,000 soldiers and 25,000 civilians died in defense of the capital, while 50,000 Warsaw inhabitants were wounded). After defeat at the final Battle of Kock, on October 2-5, 1939, the Independent Operational Group "Polesie", under the command of General Franciszek Kleeberg, surrendered.
Both sides of the military conflict suffered loss of resources and human life. Around 70,000 Polish soldiers died in battles against German forces, and around 133,000 were wounded. Meanwhile, German forces suffered around 45,000 deaths, injuries, and disappearances. On September 28, 1939, Germany and the USSR signed a document, which placed the new border along the rivers of Pisa-Narew-Bug-San. The Nazis formed new administrative units on the seized territory: Reichsgau Wartheland, which covered Greater Poland and Silesia, as well as Danzig-West Prussia, which was directly incorporated into the Reich and underwent a sharp process of Germanization and an attempt at complete elimination of Polish culture. The rest of the territory became the General Government, with Cracow as the capital. Here, Polish people - according to the concept of untermensch ("inferior people") - were to be turned into unskilled workers with a limited education, taught to be obedient to the imposed system of severe repressive measures, which was present in the "living space" of Nazi Germany.