1 September
The German attack on Poland and the beginning of the Second World War. At 4.45 am, the German battleship Schleswig-Holstein began shelling Westerplatte, the Military Transit Depot in the Free City of Gdańsk, defended by a garrison (about 200 soldiers) under the command of Maj. Henryk Sucharski and Cpt. Franciszek Dąbrowski. For seven days, the Poles heroically repelled repeated German attacks from the sea, land and air, becoming a symbol of Polish resistance. The defense of the Polish Post in Gdańsk (commanded by Konrad Guderski). The Poles surrender in the afternoon, when the mail building is set on fire. A month later they are shot by the Germans. Operation "Tannenberg". The organized and planned German murder of civilians, especially the Polish elite in the western territories of the Republic of Poland, is carried out by the Selbstschutz (sabotage units recruited from members of German minority living in Poland) and security police (the so-called Einsatzgruppen). The bombing of Wieluń. One of the first attacked towns is Wieluń, bombed by the Luftwaffe - the number of victims is estimated at around 1,200 civilians.
2 September
The establishment in Stutthof in the annexed Free City of Gdańsk the camp facility initially intended for arrested representatives of the Polish population of Gdańsk and Pomerania.
3 September
The declaration of war on the Third Reich by France and the United Kingdom. Mass manifestations of overjoyed people of Warsaw in front of the embassies of these countries. In the south of the country near Węgierska Górka the fortified area, commanded by Cpt. Tadeusz Semik, filling scarce fortifications for over 20 hours repels the attacks of the 7th Infantry Division. This facilitates the reverse maneouver of the key Kraków Army.
4 September
The beginning of the evacuation of Polish state authorities, as well as of the country’s gold from the Polish Bank in Warsaw.
5 September
The Germans enter Bydgoszcz. The beginning of mass executions of Poles (approx. 1,600 people were shot) in retaliation for quelling the diversion of ethnic Germans in the city.
7 September
Westerplatte surrenders. Photographs from the capitulation are published in German war publications for propaganda purposes among other images of the campaign in Poland testifying to the Wehrmacht's allegedly chivalrous way of waging war.
8 September
Stefan Starzyński, Mayor of the capital city of Warsaw, is nominated as Civilian Commissar at the Command of the Defense of the Capital. The beginning of the defense of Warsaw organized by Gen. Walerian Czuma. Gen. Juliusz Rómmel (whose Łódź Army had been defeated) takes the command of the newly formed Warszawa Army, its tasks including the defense of the capital city. A German Panzer Division reaches the outskirts of Warsaw in the region of Ochota and tries to seize the city on the spot. The Ciepielów Massacre. A Wermacht unit (from 15th Motorized Infantry Regiment, 29th Motorized Infantry Division) murders several dozen Polish POWs.
10 September
The fall of the fortified area of Wizna upon the Narew River, the so-called Polish Thermopylae, and the suicide death of a heroic officer, Cpt. Władysław Raginis. The positions are attacked by the 19th Army Corps under the command of the well-known theoretician and practitioner of the art of war, Gen. Heinz Guderian. The Polish troops advancing over the Bzura crush the German 30 Infantry Division, taking approx. 1,000 German POWs and seizing a number of towns (including Łęczyca).
12 September
The establishment of the Polish military Camp Coëtquidan. Soon to become the centre of formation of the Polish army in France.
13 September
The bombing of Frampol which virtually destroys this small town in Roztocze.
14 September
ORP Orzeł flows into the Estonian port of Tallinn to deliver its sick commander to hospital, and is insidiously interned there under Soviet and German pressure
15 September
Hitler appoints Hans Frank as the administrator of the occupied Polish territories.
17 September
The beginning of the Soviet aggression on Poland. Soviet troops meet the resistance of the Border Guard Corps (KOP), including "Sarny" Regiment, "Małyńsk" Grouping, "Kleck", "Ludwik", "Sienkiewicze", "Dawigródek" battalions, and Polish Army units (often formed ad hoc, e.g. Reserve Cavalry Brigade Wołkowysk) in many points along the border and deep within the Polish territory, including near Kowel, Sarny, Baranowicze, Dubno, Orany and Tarnopol. In the organization of Polish defense a special role is played by the Deputy Commander of the Border Guard, General Wilhelm Orlik-Rückemann. In the evening, the President of the Republic of Poland, Ignacy Mościcki, and the government of the Republic of Poland cross the border with Romania. The beginning of the Soviet murders of Polish officers and intelligentsia, in which, hand in hand the units of the Soviet army and the NKVD, representatives of national minorities collaborating with the Soviet occupant take part.
19 September
The end of Polish resistance on Kępa Oksywska. The commander of the Coastal Defense, Col. Stanisław Dąbek, commanding about 20 soldiers and officers, fights to the end. Wounded, he refuses to go into captivity, and commits suicide.
21 September
After the dramatic two-day combat, in which the Soviet army lost several dozen tanks and several hundred soldiers, the defense of Grodno ends. In retaliation, mass murders of the Polish population and prisoners of war are committed.
22 September
The surrender of Lwów to the Red Army. The Soviets guaranteed Polish Army officers "personal freedom and inviolability of their personal property", as well as the possibility of going abroad. These were all empty promises. Most of the officers, defenders of Lwów, found themselves in the camp in Starobielsk, and then they were murdered in Kharkiv.
27 September
The beginning of the capitulation talks. Gen. Tadeusz Kutrzeba agrees the terms of capitulation of Warsaw with the commander of the German 8th Army, General Johannes Blaskowitz. Gen. Walerian Czuma issues an order to stop fighting in Warsaw. The defence of the capital ends. In Warsaw, a military underground organization is established under the name of the Polish Victory Service (later transformed into the Union of Armed Struggle, ultimately to become the Home Army). It is headed by Gen. Michał Karaszewicz-Tokarzewski
28 September
The signing in Moscow of a treaty on borders and friendship between the Third Reich and the USSR along with a secret protocol establishing the borders between the two countries. Three additional protocols were added to the agreement, the third one containing the following statement: „Both parties will tolerate no Polish agitation in their territories which affects the territories of the other party. They will suppress in their territories all beginnings of such agitation and inform each other concerning suitable measures for this purpose.” (Secret Additional Protocol to German-Soviet Treaty on Borders and Friendship) The signing of the act of the capitulation of Warsaw.
2 October
The capitulation of Hel.
5 October
The last day of fighting near Kock. Hitler reviews a parade of German troops in Aleje Ujazdowskie in Warsaw. The planned attack on the leader of the Third Reich is cancelled.
6 October
The capitulation of the Independent Operational Group “Polesie” (SGO “Polesie”).