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.

[20] 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.

[25][26] Poland would be granted territory to its northeast in Ukraine and Belarus if it agreed to wage war against the Soviet Union, but the concessions the Poles were expected to make meant that their homeland would become largely dependent on Germany, functioning as little more than a client state.

[37] On 31 March 1939, Poland formed a military alliance with the United Kingdom and with France, believing that Polish independence and territorial integrity would be defended with their support if it were to be threatened by Germany.

[38] On the other hand, British Prime Minister Neville Chamberlain and his Foreign Secretary, Lord Halifax, still hoped to strike a deal with Hitler regarding Danzig (and possibly the Polish Corridor).

[16] With the surprise signing of the Molotov–Ribbentrop Pact on 23 August, the result of secret Nazi–Soviet talks held in Moscow, Germany neutralized the possibility of Soviet opposition to a campaign against Poland and war became imminent.

However, there was one exception: during the night of 25–26 August, a German sabotage group, which had not received orders to suspend action, attacked the Jablunkov Pass and Mosty railway station in Silesia.

[i] On 26 August, Hitler tried to dissuade the British and the French from interfering in the upcoming conflict, even pledging that the Wehrmacht forces would be made available to Britain's empire in the future.

However, he was pressured into revoking the order by the French, who apparently still hoped for a diplomatic settlement, failing to realize that the Germans were fully mobilized and concentrated at the Polish border.

In late 1938, the Polish Air Force also ordered 300 advanced PZL.46 Sum light bombers, but due to a delay in starting mass production, none of them were delivered before 1 September.

[70] In case of a failure to defend most of the territory, the army was to retreat to the south-east of the country, where the rough terrain, the Stryj and Dniestr rivers, valleys, hills and swamps would provide natural lines of defence against the German advance, and the Romanian Bridgehead could be created.

[80] As the prospect of conflict increased, the British government pressed Marshal Edward Śmigły-Rydz to evacuate the most modern elements of the Polish Navy from the Baltic Sea.

At 04:45, the old German pre-dreadnought battleship Schleswig-Holstein opened fire on the Polish military transit depot and coastal fort at Westerplatte, in the Free City of Danzig, on the Baltic Sea.

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.

[103] In the last week of September, Hitler made a speech in Danzig and said: Meantime, Russia felt moved, on its part, to march in for the protection of the interests of the White Russian and Ukrainian people in Poland.

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.

[113] During Operation Tannenberg, a campaign of ethnic cleansing organized by multiple elements of the German government, tens of thousands of Polish civilians were shot at 760 mass execution sites by the Einsatzgruppen.

[117] On 3 October 1939, the military districts centered on and named "Lodz" and "Krakau" were set up under command of major generals Gerd von Rundstedt and Wilhelm List, and Hitler appointed Hans Frank and Arthur Seyß-Inquart as civil heads, respectively.

[117] 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.

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.

[citation needed] 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".

In any case, this line exists and, consequently, the Eastern front has been created, which Nazi Germany will not dare to attack...[126]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.

[130] Among the 100,000 people murdered in the Intelligenzaktion operations in 1939–1940, approximately 61,000 were members of the Polish intelligentsia: scholars, clergy, former officers, and others, whom the Germans identified as political targets in the Special Prosecution Book-Poland, compiled before the war began in September 1939.

Italian reporter Indro Montanelli promptly published an article in the Corriere della Sera, on the brave and heroic Polish cavalry men, who charged German tanks with sabres and lances.

While exact estimates vary, Poland cost the Germans about 45,000 casualties and 11,000 damaged or destroyed military vehicles, including 993 tanks and armored cars, 565 to 697 airplanes and 370 artillery pieces.

[143][144][145] As for duration, the September Campaign lasted about a week and a half less than the Battle of France in 1940 even though the Anglo-French forces were much closer to parity with the Germans in numerical strength and equipment and were supported by the Maginot line.

Many early post-war histories, such as Barrie Pitt's in The Second World War (BPC Publishing 1966), attribute German victory to "enormous development in military technique which occurred between 1918 and 1940", and cite that "Germany, who translated (British inter-war) theories into action... called the result Blitzkrieg".

Matthew Cooper writes: Throughout the Polish Campaign, the employment of the mechanized units revealed the idea that they were intended solely to ease the advance and to support the activities of the infantry....

The paralysis of command and the breakdown of morale were not made the ultimate aim of the ... German ground and air forces, and were only incidental by-products of the traditional manoeuvers of rapid encirclement and of the supporting activities of the flying artillery of the Luftwaffe, both of which had as their purpose the physical destruction of the enemy troops.

They argue that Western accounts of the September campaign have stressed the shock value of the panzers and Stuka attacks have "tended to underestimate the punishing effect of German artillery on Polish units.