When the German army and air force attacked in the early hours of 1 September 1939 it was difficult to predict how long the Poles would be able to hold out. Initially, the Polish political and military leaders were convinced that, if in due course they obtained British and French assistance, they would be able to create a credible Eastern front. But by 5 September they were more preoccupied with leaving Poland and transforming themselves into a leadership-in-exile. The immediate fate of Poland, and the Polish people facing the onslaught of the German troops, became a matter of secondary importance.

During the course of their joint military talks in May 1939 neither British nor French military leaders had credited the Polish army with an ability to withstand a full-scale German attack. They were, of course, thinking of the Polish army's organization, its strategic thinking and degree of preparedness, and the supplies available to it. They did not doubt the Polish leadership's commitment to fighting, its bravery and most of all its political unity and organizational skills. These turned out to be as much a source of Polish military weakness as was the general Polish unreadiness to face the German attack. In the years to come these problems of the government-in-exile's political disunity and organizational ineptitude were to remain unresolved.

The Polish–German war was not concluded by a negotiated armistice. This fact itself moulded the mentality of the Poles who subsequently went into exile.