Mediterranean and Middle East. German forces invaded Poland from the north, south, and west the morning after the Gleiwitz incident. Slovak military forces advanced alongside the Germans in northern Slovakia.
As the Wehrmacht advanced, Polish forces withdrew from their forward bases of operation close to the Germany—Poland border to more established defense lines 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.
France invaded a small part of Germany in the Saar Offensive , and the Polish army was effectively defeated even before the British Expeditionary Force could be transported to Europe, with the bulk of the BEF in France by the end of September.
On 17 September, the Soviet Red Army invaded Eastern Poland , the territory beyond the Curzon Line that fell into the Soviet " sphere of influence " according to the secret protocol of the Molotov—Ribbentrop Pact; this rendered the Polish plan of defence obsolete.
