Invasion of Poland

The morning after the Gleiwitz incident, German forces invaded Poland from the north, south, and west. As the Germans advanced, Polish forces withdrew from their forward bases of operation close to the Polish–German border to more established lines of defence to the east. After the mid-September Polish defeat in the Battle of the Bzura, the Germans gained an undisputed advantage. Polish forces then withdrew to the southeast where they prepared for a long defence of the Romanian Bridgehead and awaited expected support and relief from France and the United Kingdom. Those two countries had pacts with Poland and had declared war on Germany on 3 September, though in the end their aid to Poland in the September campaign was very limited.

The Soviet Red Army's invasion of Eastern Poland on 17 September, in accordance with a secret protocol of the Molotov–Ribbentrop Pact, rendered the Polish plan of defence obsolete. Facing a second front, the Polish government concluded the defence of the Romanian Bridgehead was no longer feasible and ordered an emergency evacuation of all troops to neutral Romania. On 6 October, following the Polish defeat at the Battle of Kock, German and Soviet forces gained full control over Poland. The success of the invasion marked the end of the Second Polish Republic, though Poland never formally surrendered.

On 8 October, after an initial period of military administration, Germany directly annexed western Poland and the former Free City of Danzig and placed the remaining block of territory under the administration of the newly established General Government. The Soviet Union incorporated its newly acquired areas into its constituent Belarusian and Ukrainian republics, and immediately started a campaign of sovietization. In the aftermath of the invasion, a collective of underground resistance organizations formed the Polish Underground State within the territory of the former Polish state. Many of the military exiles that managed to escape Poland subsequently joined the Polish Armed Forces in the West, an armed force loyal to the Polish government in exile.

Details of the campaign

Phase 1: German invasion

Following several German-staged incidents (like the Gleiwitz incident, a part of Operation Himmler), which German propaganda used as a pretext to claim that German forces were acting in self-defence, the first regular act of war took place on 1 September 1939, at 04:40, when the Luftwaffe attacked the Polish town of Wieluń, destroying 75% of the city and killing close to 1,200 people, most of them civilians. This invasion subsequently began World War II. Five minutes later, the old German pre-dreadnought battleship Schleswig-Holstein opened fire on the Polish military transit depot at Westerplatte in the Free City of Danzig on the Baltic Sea. At 08:00, German troops—still without a formal declaration of war issued—attacked near the Polish town of Mokra. The Battle of the Border had begun. Later that day, the Germans attacked on Poland's western, southern and northern borders, while German aircraft began raids on Polish cities. The main axis of attack led eastwards from Germany proper through the western Polish border. Supporting attacks came from East Prussia in the north, and a co-operative German-Slovak tertiary attack by units (Field Army "Bernolák") from German-allied Slovakia in the south. All three assaults converged on the Polish capital of Warsaw.

The Allied governments declared war on Germany on 3 September; however, they failed to provide any meaningful support. The German-French border saw only a few minor skirmishes, although the majority of German forces, including 85% of their armoured forces, were engaged in Poland. Despite some Polish successes in minor border battles, German technical, operational and numerical superiority forced the Polish armies to retreat from the borders towards Warsaw and Lwów. The Luftwaffe gained air superiority early in the campaign. By destroying communications, the Luftwaffe increased the pace of the advance which overran Polish airstrips and early warning sites, causing logistical problems for the Poles. Many Polish Air Force units ran low on supplies, 98 of their number withdrew into then-neutral Romania. The Polish initial strength of 400 was reduced to just 54 by 14 September and air opposition virtually ceased.

By 3 September, when Günther von Kluge in the north had reached the Vistula River (some 10 km (6.2 mi) from the German border at that time) and Georg von Küchler was approaching the Narew River, Walther von Reichenau's armor 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. By 8 September, one of his armored corps—having advanced 225 km (140 mi) in the first week of the campaign—reached the outskirts of Warsaw. 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 San River 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 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 launching disjointed attacks on the nearest German columns.

Polish forces abandoned the regions of Pomerelia (the Polish Corridor), Greater Poland and Polish Upper Silesia in the first week. The Polish plan for border defence was proven a dismal failure. The German advance as a whole was not slowed. 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 encirclement of the Polish forces west of the Vistula (in the Łódź area and, still farther west, around Poznań) and also penetrating deeply into eastern Poland. Warsaw—under heavy aerial bombardment since the first hours of the war—was attacked on 9 September and was put under siege on 13 September. Around that time, advanced German forces also reached the city of Lwów, a major metropolis in eastern Poland. 1,150 German aircraft bombed Warsaw on 24 September.

The Polish defensive plan called for a strategy of encirclement: they were to allow the Germans to advance in between two Polish Army groups in the line between Berlin and Warsaw-Lodz, at which point Armia Prusy would move in and repulse the German spearhead, trapping them. In order for this to happen, Armia Prusy needed to be fully mobilized by 3 September. However, Polish military planners failed to foresee the speed of the German advance and predicted that Armia Prusy would need to be fully mobilized by 16 September, by which time it was too late.

The largest battle during this campaign—the Battle of Bzura—took place near the Bzura river west of Warsaw and lasted 9–19 September. Polish armies Poznań and Pomorze, retreating from the border area of the Polish Corridor, attacked the flank of the advancing German 8th Army, but the counterattack failed after initial success. After the defeat, Poland lost its ability to take the initiative and counterattack on a large scale. German air power was instrumental during the battle. The Luftwaffe's offensive broke what remained of Polish resistance in an "awesome demonstration of air power". The Luftwaffe quickly destroyed the bridges across the Bzura River. Afterward, the Polish forces were trapped out in the open, and were attacked by wave after wave of Stukas, dropping 50 kg (110 lb) "light bombs" which caused huge numbers of casualties. The Polish anti-aircraft batteries ran out of ammunition and retreated to the forests, but were then "smoked out" by the Heinkel He 111 and Dornier Do 17s dropping 100 kg (220 lb) incendiaries. The Luftwaffe left the army with the task of mopping up survivors. The Stukageschwaders alone dropped 388 t (428 short tons) of bombs during this battle.

The Polish government (of President Ignacy Mościcki) and the high command (of Marshal Edward Rydz-Śmigły) left Warsaw in the first days of the campaign and headed southeast, reaching Lublin on 6 September. From there, it moved on 9 September to Kremenez, and on 13 September to Zaleshiki on the Romanian border. Rydz-Śmigły ordered the Polish forces to retreat in the same direction, behind the Vistula and San rivers, beginning the preparations for the long defense of the Romanian Bridgehead area.

Phase 2: after Soviet Union invasion from the east

From the beginning, the German government repeatedly asked Vyacheslav Molotov whether the Soviet Union would keep to its side of the partition bargain. The Soviet forces were holding fast along their designated invasion points pending finalization of the five-month-long undeclared war with Japan in the Far East. On 15 September 1939 the Ambassadors Molotov and Shigenori Tōgō completed their agreement ending the conflict, and the Nomonhan cease-fire went into effect on 16 September 1939. Now cleared of any "second front" threat from the Japanese, Soviet premier Joseph Stalin ordered his forces into Poland on 17 September. It was agreed that the USSR would relinquish its interest in the territories between the new border and Warsaw in exchange for inclusion of Lithuania in the Soviet "zone of interest".

By 17 September, the Polish defence was already broken and the only hope was to retreat and reorganize along the Romanian Bridgehead. However, these plans were rendered obsolete nearly overnight, when the over 800,000-strong Soviet Red Army entered and created the Belarussian and Ukrainian fronts after invading the eastern regions of Poland in violation of the Riga Peace Treaty, the Soviet-Polish Non-Aggression Pact, and other international treaties, both bilateral and multilateral. Soviet diplomacy claimed that they were "protecting the Ukrainian and Belarusian minorities of eastern Poland since the Polish government had abandoned the country and the Polish state ceased to exist".

Polish border defence forces in the east—known as the Korpus Ochrony Pogranicza—consisted of about 25 battalions. Edward Rydz-Śmigły ordered them to fall back and not engage the Soviets. This, however, did not prevent some clashes and small battles, such as the Battle of Grodno, as soldiers and local population attempted to defend the city. The Soviets murdered numerous Polish officers, including prisoners of war like General Józef Olszyna-Wilczyński. The Organization of Ukrainian Nationalists rose against the Poles, and communist partisans organized local revolts, robbing and murdering Poles. Those movements were quickly disciplined by the NKVD. The Soviet invasion was one of the decisive factors that convinced the Polish government that the war in Poland was lost. Prior to the Soviet attack from the east, the Polish military's fall-back plan had called for long-term defence against Germany in the south-eastern part of Poland, while awaiting relief from a Western Allies attack on Germany's western border. However, the Polish government refused to surrender or negotiate a peace with Germany. Instead, it ordered all units to evacuate Poland and reorganize in France.

Meanwhile, Polish forces tried to move towards the Romanian Bridgehead area, still actively resisting the German invasion. From 17–20 September, Polish armies Kraków and Lublin were crippled at the Battle of Tomaszów Lubelski, the second-largest battle of the campaign. The city of Lwów capitulated on 22 September because of Soviet intervention; the city had been attacked by the Germans over a week earlier, and in the middle of the siege, the German troops handed operations over to their Soviet allies. Despite a series of intensifying German attacks, Warsaw—defended by quickly reorganized retreating units, civilian volunteers and militia—held out until 28 September. The Modlin Fortress north of Warsaw capitulated on 29 September after an intense 16-day battle. Some isolated Polish garrisons managed to hold their positions long after being surrounded by German forces. Westerplatte enclave's tiny garrison capitulated on 7 September and the Oksywie garrison held until 19 September; Hel Fortified Area was defended until 2 October. In the last week of September, Hitler made a speech in the city of Danzig in which he said:

Poland never will rise again in the form of the Versailles treaty. That is guaranteed not only by Germany, but also… Russia.

Despite a Polish victory at the Battle of Szack, after which the Soviets executed all the officers and NCOs they had captured, the Red Army reached the line of rivers Narew, Bug River, Vistula and San by 28 September, in many cases meeting German units advancing from the other direction. Polish defenders on the Hel peninsula on the shore of the Baltic Sea held out until 2 October. The last operational unit of the Polish Army, General Franciszek Kleeberg's Samodzielna Grupa Operacyjna "Polesie", surrendered after the four-day Battle of Kock near Lublin on 6 October marking the end of the September Campaign.

Civilian losses

The Polish September Campaign was an instance of total war. Consequently, civilian casualties were high during and after combat. From the start, the Luftwaffe attacked civilian targets and columns of refugees along the roads to wreak havoc, disrupt communications, and target Polish morale. Apart from the victims of battles, the German forces (both SS and the regular Wehrmacht) murdered several thousand Polish civilians. During Operation Tannenberg, nearly 20,000 Poles were shot at 760 mass execution sites by the Einsatzgruppen.

The Polish Campaign was the first action by Adolf Hitler in his attempt to create Lebensraum, or living space, for the ethnic German people. The German retaliation against the opposing Polish civilians quickly turned into an atrocity, an irregular warfare against Polish men, women, and children. The brutality that the German army carried out on the civilians was justifiable in their eyes. Many historians have studied the reasoning behind this brutality and have found that Nazi propaganda could be one of the factors. Nazi propaganda worked to manipulate the German people into believing that the Jewish and other ethnic people were the enemy.

Altogether, the civilian losses of Polish population amounted to about 150,000–200,000 while German civilian losses amounted to roughly 3,250 (including 2,000 who died fighting Polish troops as members of a fifth column).

Aftermath

Poland was divided among Germany, the Soviet Union, and Slovakia. Lithuania received the city of Vilnius and its environs on 28 October 1939 from the Soviet Union. On 8 and 13 September 1939, the German military districts of "Posen" (Poznan)—commanded by General Alfred von Vollard-Bockelberg—and "Westpreußen" (West Prussia)—commanded by General Walter Heitz—were established in conquered Greater Poland and Pomerelia, respectively. Based on laws of 21 May 1935 and 1 June 1938, the German Wehrmacht shared its administrative powers with civilian "chief civil administrators" (Chefs der Zivilverwaltung, CdZ). German dictator Adolf Hitler appointed Arthur Greiser to become the CdZ of the Posen military district, and Danzig's Gauleiter Albert Forster to become the CdZ of the West Prussian military district. On 3 October, the military districts "Lodz" and "Krakau" (Cracow) were set up under command of Generalobersten (Colonel-Generals) Gerd von Rundstedt and Wilhelm List, and Hitler appointed Hans Frank and Arthur Seyss-Inquart as civil heads, respectively. At the same time, Frank was appointed "supreme chief administrator" for all occupied territories. On 28 September, another secret German-Soviet protocol modified the arrangements of August: all of Lithuania was shifted to the Soviet sphere of influence; in exchange, the dividing line in Poland was moved in Germany's favour, eastwards towards the Bug River. On 8 October, Germany formally annexed the western parts of Poland with Greiser and Forster as Reichsstatthalter, while the south-central parts were administered as the General Government led by Frank.

Even though water barriers separated most of the spheres of interest, the Soviet and German troops met on numerous occasions. The most remarkable event of this kind occurred at Brest-Litovsk on 22 September. The German 19th Panzer Corps—commanded by General Heinz Guderian—had occupied the city, which lay within the Soviet sphere of interest. When the Soviet 29th Tank Brigade—commander by S. M. Krivoshein—approached, the commanders negotiated that the German troops would withdraw and the Soviet troops would enter the city saluting each other. At Brest-Litovsk, Soviet and German commanders held a joint victory parade before German forces withdrew westward behind a new demarcation line. Just three days earlier, however, the parties had a more hostile encounter near Lwow (Lviv, Lemberg), when the German 137th Gebirgsjägerregimenter (mountain infantry regiment) attacked a reconnaissance detachment of the Soviet 24th Tank Brigade; after a few casualties on both sides, the parties turned to negotiations. The German troops left the area, and the Red Army troops entered Lviv on 22 September.

The Molotov–Ribbentrop pact and the invasion of Poland marked the beginning of a period during which the government of the Soviet Union increasingly tried to convince itself that the actions of Germany were reasonable, and were not developments to be worried about, despite evidence to the contrary. On 7 September 1939, just a few days after France and Britain joined the war against Germany, Stalin explained to a colleague that the war was to the advantage of the Soviet Union, as follows:

A war is on between two groups of capitalist countries... for the redivision of the world, for the domination of the world! We see nothing wrong in their having a good hard fight and weakening each other... Hitler, without understanding it or desiring it, is shaking and undermining the capitalist system... We can manoeuvre, pit one side against the other to set them fighting with each other as fiercely as possible... The annihilation of Poland would mean one fewer bourgeois fascist state to contend with! What would be the harm if as a result of the rout of Poland we were to extend the socialist system onto new territories and populations?

About 65,000 Polish troops were killed in the fighting, with 420,000 others being captured by the Germans and 240,000 more by the Soviets (for a total of 660,000 prisoners). Up to 120,000 Polish troops escaped to neutral Romania (through the Romanian Bridgehead and Hungary), and another 20,000 to Latvia and Lithuania, with the majority eventually making their way to France or Britain. Most of the Polish Navy succeeded in evacuating to Britain as well. German personnel losses were less than their enemies (~16,000 KIA).

None of the parties to the conflict—Germany, the Western Allies or the Soviet Union—expected that the German invasion of Poland would lead to a war that would surpass World War I in its scale and cost. It would be months before Hitler would see the futility of his peace negotiation attempts with the United Kingdom and France, but the culmination of combined European and Pacific conflicts would result in what was truly a "world war". Thus, what was not seen by most politicians and generals in 1939 is clear from the historical perspective: The Polish September Campaign marked the beginning of the Second World War in Europe, which combined with the Japanese invasion of China in 1937 and the Pacific War in 1941, formed the cataclysm known as World War II.

The invasion of Poland led Britain and France to declare war on Germany on 3 September. However, they did little to affect the outcome of the September Campaign. No declaration of war was issued by Britain and France against the Soviet Union. This lack of direct help led many Poles to believe that they had been betrayed by their Western allies.

On 23 May 1939, Hitler explained to his officers that the object of the aggression was not Danzig, but the need to obtain German Lebensraum and details of this concept would be later formulated in the infamous Generalplan Ost. The invasion decimated urban residential areas, civilians soon became indistinguishable from combatants, and the forthcoming German occupation (both on the annexed territories and in the General Government) was one of the most brutal episodes of World War II, resulting in between 5.47 million and 5.67 million Polish deaths (about 20% of the country's "total" population, and over 90% of its Jewish minority)—including the mass murder of 3 million Polish citizens (mainly Jews as part of the final solution) in extermination camps like Auschwitz, in concentration camps, and in numerous ad hoc massacres, where civilians were rounded up, taken to a nearby forest, machine-gunned, and then buried, whether they were dead or not.

According to the Polish Institute of National Remembrance, Soviet occupation between 1939 and 1941 resulted in the death of 150,000 and deportation of 320,000 of Polish citizens, when all who were deemed dangerous to the Soviet regime were subject to sovietization, forced resettlement, imprisonment in labor camps (the Gulags) or murdered, like the Polish officers in the Katyn massacre.