During the time Józef Piłsudski was the dictator of Poland, most of Polish planning concentrated on contingences in case of a possible attack from the East. It was only after Piłsudski's death in 1935 that the new Polish government and military reevaluated the situation and decided that the current Polish plan for a Polish-German war, dating from the mid-1920s (Plan "S"), was inadequate and needed to be revised. However up to 1938, the priority was war in the East, not the West, and a majority of Polish fortifications were being erected on the Polish-Soviet border.
The first version predicted that Germans would attack from Pomerania towards Warsaw, with supporting thrusts from Silesia and Prussia, aiming at establishing an early link through the Polish Corridor between German Pomerania and Prussia. After German annexation of parts of Czechoslovakia and changes of borders, Polish planners revised the plan with the expectation that a main thrust would originate from Silesia - through Piotrków and Łódź towards Warsaw and Kraków. The Polish planners correctly predicted the direction of most German thrusts, with one crucial exception: they assigned low priority to a possible deep, flanking, eastward push from Prussia and Slovakia, a push that was however assigned high priority in the German plan (Fall Weiss).
A controversy involved the decision whether Polish forces should defend the lengthy borders, or withdraw east and south and try a defense along a shorter line, backed with rivers. Although the second plan was more militarily sound, political considerations outweighed them, as Polish politicians were concerned that Germany could be satisfied with occupation of some disputed territories (like the Free City of Danzig, the Polish Corridor and Silesia), and push for an early end of the war after occupying those territories. The western regions were also the most densely populated and had major industrial centers, crucial for mobilization and any continued military production of equipment and supply for the Polish Army.
Even with the decision to protect the borders, due to Poland being virtually encircled from three sides by the Germans, it was decided that some areas had to be abandoned early on, as their defence would be next to impossible. Thus the north-west Pomorze Voivodship and Poznań Voivodship were to be abandoned early on, with a separate force, the Land Coastal Defence protecting key parts of the coast as long as possible, and most of the surface Polish Navy evacuated to the United Kingdom as specified in the Peking Plan (submarines were to engage the enemy in the Baltic Sea as per the Worek Plan). The main Polish defence line was to be formed on the regions of the Augustów Primeval Forest - Biebrza River - Narew River - Vistula River (and the towns of Modlin, Toruń, Bydgoszcz) - Inowrocław lakes - Warta River - Widawka River - town of Częstochowa - Silesian fortifications - town of Bielsko-Biała - town of Żywiec - village of Chabówka - and the town of Nowy Sącz). The second defensive line was based on the Augustów Forest - Biebrza River - Narew River - Bug River - Vistula River - and Dunajec River. Finally, the third defensive line involved retreating southeast towards the Romanian border, and holding as long as possible in the Romanian bridgehead region.
The plan assumed the Soviet Union would be neutral, as a Nazi-Soviet alliance seemed unlikely. The plan however allowed for a Lithuanian attempt to take Wilno, a city disputed between Poland and Lithuania, and a small Polish force - primarily elite units of Border Defence Corps - was detached to secure that region.
The plan assumed that Polish forces would be able to hold for several months but due to German numerical and technical superiority would be pushed back (it was estimated Germans would have two to threefold advantage), until pressure from Western Allies (France and United Kingdom) who were obliged (through the Franco-Polish Military Alliance and Polish-British Common Defence Pact) to launch an offensive from the West would draw enough German forces away from the Polish front to allow Polish forces to launch a counteroffensive.
The plan correctly assumed the size, location and most directions of attack by the enemy. By the time of the German attack, however, the second and further defensive lines and related items were not fully defined by the plan, nor had any of its aspects been subject to a military exercise. There were also other unfinished parts, particularly dealing with communications and supplies.
When Germany invaded Poland on 1 September 1939, Polish forces were dealt a significant defeat at the Battle of the Border, just as the critics of the plan predicted. Further factors, such as underestimating German mobility and blitzkrieg strategy, and overestimating Polish mobility, the Soviet invasion of Poland and lack of promised aid from the Western Allies, contributed to the Polish forces' defeat by 6 October 1939.
- Plan Wschód (Plan East), a Polish defensive plan in case of an attack by the Soviet Union
- Seidner, Stanley S. (1978), Marshal Edward Śmigły-Rydz Rydz and the Defense of Poland, New York, OCLC 164675876
- Dunn, John P. (1987), Polish Defense Planning, 1919-1939: Myth vs. Reality (Dissertation), Florida Atlantic University, OCLC 18811808
- (Polish) Plan "Zachód"