'Zachód' was the Polish defence plan against a German invasion (1939).

Unfortunately for the chances of Poland’s survival against any such attack, the plan was based on the political rather than military decision that it was essential to deploy Polish forces right up against the frontier with Germany. The decision was taken for national reasons, but strengthened by the British promise to come to Poland’s military aid in the event of a German invasion. Given that Poland’s most valuable natural resources, the bulk of its industry and its most densely populated regions were all close to the country’s western border with Germany in the region of Silesia, Polish policy was perhaps inevitably centred on the protection of these regions, the more so as many politicians feared that if Poland should retreat from the regions disputed by Germany, the UK and France would sign a separate peace treaty with Germany along the lines of the Munich agreement of 1938, which had allocated the Sudetenland region of Czechoslovakia to Germany in return for German promises of no further demands.

Neither France nor the UK had specifically guaranteed the borders or territorial integrity of Poland, moreover, so the Polish leadership therefore felt that it had to disregard the French military advice to deploy the bulk of Poland’s forces behind the natural defensive barriers provided by the wide Vistula and San rivers, even though some Polish generals supported it as a better strategy in purely military terms.

The 'Zachód' plan did envisage a Polish retreat farther into the country, but only in the form of a slow withdrawal to prepared positions near the country’s great waterways (the Narew, Vistula and San rivers), giving the country time to finish its mobilisation, and was to be turned into a general counter-offensive when the Western Allies launched their own promised offensive.

The worst-case scenario envisaged by the Polish army was a retreat behind the San river into the south-eastern provinces, where an extended defence was envisaged within the so-called 'Romanian bridgehead'. 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 its Western Allies would keep their end of the alliance treaty and quickly start an offensive of their own to relieve the pressure of Poland. In the event neither the French nor the British made more than even the most limited plans to attack Germany while the Polish campaign was being fought. In addition, the British and French expected a Polish campaign to develop into trench warfare, forcing the Germans to sign a peace treaty restoring Poland’s pre-war borders. 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 for a cordon defence of the western borders contributed vastly to the eventual defeat of Poland. The Polish forces were stretched thinly along the very long border without compact defence lines and good defensive positions along advantageous terrain, so Germany’s mechanised forces were often able to encircle them. The Polish reliance on tactically poor forward defences lacking in depth was also reliant on lines of communication which were often poorly protected.

About one-third of Poland’s forces were concentrated in or near the Polish 'corridor' to the Baltic Sea in north-western Poland, and here they were exposed to a double envelopment from East Prussia and from the west, and thus isolation in a pocket. In the south, facing the main avenues of a German advance, the Polish forces were much more 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 the control of the Polish commander, Marszałek Połski Edward Śmigły-Rydz.

The Poles' decision in favour of a forward deployment effectively sacrificed all chance of fighting a series of delaying actions, since the vast majority of their army was limited to movement on foot, and was therefore unable to retreat to defensive positions in the rear before these were overrun by German mechanised and motorised columns.

The political decision to defend the border was not Poland’s sole strategic error. Polish pre-war propaganda had always stressed that any German invasion would be defeated with ease, so that the eventual Polish defeats in the 'Weiss' (i) campaign of September 1939 came as a huge psychological and morale shock to the bulk of the civilian population which, wholly unprepared for such news and with no training for such an event, panicked and retreated to the east, spreading chaos, lowering troop morale and making the road movement of Polish troops very difficult. The propaganda also had some negative consequences for the Polish troops, whose communications, disrupted by German mechanised and motorised formations in their rear and the panic-struck movement of civilians, 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 counterattacking or would soon receive reinforcements from other victorious areas.