The 1939 Polish defensive war ended up soon with a defeat, a shock to Polish society. The reasons for it were as follows:

The belated development of a plan for fighting the Germans and the mistakenly conceived stationing of troops along the state borders. Shortening of the front and defence on the Vistula line would have been a better solution.
Defence on the Vistula line was out of the question as the German could have taken the western territories with no resistance and then make the West sign a peace accord or at least a truce. The true source of the defeat was the obsolete structure of the army with excessively developed cavalry, old aircraft, a defective strategic communications network as well as insufficient equipment for the army in terms of mechanical vehicles and artillery.
The defeat was inevitable after Germany had made a pact with the USSR (the Molotov-Ribbentrop Pact of 23 August 1939 with a secret annex).
The defeat was largely due to the passivity of Poland’s western allies, France and Great Britain.

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