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German forces invaded Poland from the north, south, and west the morning after the Gleiwitz incident. Slovak forces advanced alongside the Germans in northern Slovakia.
As the Wehrmacht advanced, Polish forces withdrew from their forward bases of operation close to the Polish–German border to more established lines of defence to the east. After the mid-September Polish defeat in the Battle of the Bzura, the Germans gained an undisputed advantage.
Polish forces then withdrew to the southeast where they prepared for a long defence of the Romanian Bridgehead and awaited expected support and relief from France and the United Kingdom. While those two countries had pacts with Poland and had declared war on Germany on 3 September, in the end their aid to Poland was very limited.