Invasion of Poland (1939)
The Poles realized the British and French were in a phoney war and had no intention of assisting Poland, and the Soviet Red Army invaded Poland from the east on September 17.  The Polish High Command ordered troops and equipment to the Romanian Bridgehead to enter Romania with the intention to continue their resistance by moving as much of their army to France as possible.  Most of the Polish Navy escaped to Britain.  Polish cryptographers who had cracked the German Enigma machine went to France and later to Britain, to continue their work.