Details of the campaign

German plan

The German plan Fall Weiss, for what became known as the September campaign, was created by General Franz Halder, chief of the general staff, and directed by General Walther von Brauchitsch, the commander in chief of the upcoming campaign. The plan called for the start of hostilities before the declaration of war and to pursue a traditional doctrine of mass encirclement and destruction of enemy forces, assisted by the Germans' material advantages, including the use of modern airpower and tanks. The infantry - far from completely mechanized but with fast moving artillery and logistic support - was to be supported by German tanks (panzers) and small numbers of truck-mounted infantry (the Schützen regiments, forerunners of the panzergrenadiers) to assist the rapid movement of the invaders, concentrating on localized parts of the enemy front, eventually isolating segments of the enemy, surrounding, and destroying them. The pre-war armored idea (which an American journalist in 1939 would dub Blitzkrieg), which was advocated by some generals including Guderian, would have had the armor blasting holes in the enemy's front and ranging deep into the enemy's rear areas, but in actual fact the campaign in Poland would be fought along more traditional lines. This was due to conservatism on the part of the German high command, who mainly restricted the role of armor and mechanized forces to supporting the conventional infantry divisions.

Poland was a country well suited for mobile operations, being a country of flat plains with long frontiers totalling almost 3,500 miles. Those frontiers included long borders with Germany on the west and north (facing East Prussia) of 1,250 miles. Those had been extended by another 500 miles on the southern side in the aftermath of the Munich Agreement of 1938; the German occupation of Bohemia and Moravia and creation of the German puppet state of Slovakia meant that Poland's southern flank became exposed to invasion.

German planners intended to fully utilise their advantageously long border with the great enveloping manoeuvre of Fall Weiss. German units were to invade Poland from three directions:

A main attack from the German mainland through the western Polish border. This was to be carried out by Army Group South commanded by General Gerd von Rundstedt, attacking from German Silesia and from the Moravian and Slovakian border: General Johannes Blaskowitz's 8th Army was to drive eastward against Łódź; General Wilhelm List's 14th Army was to push on toward Kraków and to turn the Poles' Carpathian flank; and General Walter von Reichenau's 10th Army, in the centre with Army Group South's armour, was to deliver the decisive blow with a northwestward thrust into the heart of Poland.
A second route of attack from the northern Prussian area. General Fedor von Bock commanded Army Group North comprising General Georg von Küchler's 3rd Army, which struck southward from East Prussia, and General Günther von Kluge's 4th Army, which struck eastward across the base of the Polish Corridor.
A tertiary attack by part of Army Group South's allied Slovak units from the territory of Slovakia.
From within Poland the German minority would assist in the assault on Poland by engaging in diversion and sabotage operations through Selbstschutz units prepared before the war.
All three assaults were to converge on Warsaw, while the main Polish army was to be encircled and destroyed west of the Vistula. Fall Weiss was initiated on 1 September 1939 and was the first operation of the Second World War in Europe.

Polish plan

The Polish defense plan, Zachód, was shaped by political determination to deploy forces directly at the German-Polish border, based upon London's promise to come to Warsaw's military aid in the event of invasion. Moreover, with the nation's most valuable natural resources, industry and highly populated regions near the western border (Silesia region), Polish policy was centered on protection of those regions, especially as many politicians feared that if Poland should retreat from the regions disputed by Germany (like the Polish Corridor, cause of the famous "Danzig or War" ultimatum), Britain and France would sign a separate peace treaty with Germany similar to the Munich Agreement of 1938, especially as none of those countries specifically guaranteed Polish borders and territorial integrity. On those grounds, Poland disregarded French advice to deploy the bulk of their forces behind the natural barriers of the wide Vistula and San rivers, even though some Polish generals supported it as a better strategy. The Zachód plan did allow the Polish armies to retreat inside the country, but it was supposed to be a slow retreat behind prepared positions near rivers (Narew, Vistula and San), giving the country time to finish its mobilisation, and was to be turned into a general counteroffensive when the Western Allies would launch their own promised offensive.

The Polish Army's most pessimistic fall-back plan involved retreat behind the river San to the southeastern voivodships and their lengthy defence (the Romanian bridgehead plan). The British and French estimated that Poland should be able to defend that region for two to three months, while Poland estimated it could hold it for at least six months. This Polish plan was based around the expectation that the Western Allies would keep their end of the signed alliance treaty and quickly start an offensive of their own. However, neither the French nor the British government had made plans to attack Germany while the Polish campaign was fought. Their plans were based on the experiences of the Great War, and they expected to wear down the Germans in trench warfare, eventually forcing them to sign a peace treaty and restore Polish independence. The Polish government, however, was not notified of this strategy and based all of its defence plans on the expectation of a quick relief action by their Western Allies.

The plan to defend the borders contributed vastly to the Polish defeat. As during the September Campaign, Polish forces were stretched thin on the very long border and, lacking compact defence lines and good defence positions, having poorly secured supply lines, were often encircled by mechanized German forces. Approximately one-third of Poland's forces were concentrated in or near the Polish Corridor (in northwestern Poland), where they were perilously exposed to a double envelopment — from East Prussia and the west combined and isolated in a pocket. In the south, facing the main avenues of a German advance, the Polish forces were thinly spread. At the same time, nearly another one-third of Poland's troops were massed in reserve in the north-central part of the country, between the major cities of Łódź and Warsaw, under commander in chief Marshal Edward Rydz-Śmigły. The Poles' forward concentration in general forfeited their chance of fighting a series of delaying actions, since their army who, unlike Germany's, still traveled largely on foot and was unable to retreat to their defensive positions in the rear or to staff them before being overrun by the invader's mechanized columns.

The political decision to defend the border was not the Polish high command's only strategic mistake. Polish pre-war propaganda stated that any German invasion would be easily repelled, so that the eventual Polish defeats in the September Campaign came as a shock to many civilians, who, unprepared for such news and with no training for such an event, panicked and retreated east, spreading chaos, lowering troop morale and making road transportation for Polish troops very difficult. The propaganda also had some negative consequences for the Polish troops themselves, whose communications, disrupted by German mobile units operating in the rear and civilians blocking roads, were further thrown into chaos by bizarre reports from Polish radio stations and newspapers, which often reported imaginary victories and other military operations. This led to some Polish troops being encircled or taking a stand against overwhelming odds, when they thought they were actually counterattacking or would soon receive reinforcements from other victorious areas.