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In 1939, Great Britain and the Second Polish Republic signed the Anglo-Polish Military Alliance. According to Article I of the agreement:
"Should one of the Contracting Parties become engaged in hostilities with a European Power in consequence of aggression by the latter against that Contracting Party, the other Contracting Party will at once give the Contracting Party engaged in hostilities all the support and assistance in its power."
Neither Great Britain nor France, which had a similar military alliance kept their word when the Soviet Union invaded Eastern Poland on September 17, 1939. The Soviet Union did not declare war, but it was certainly a European power, and it was very much "engaged in hostilities" with the forces of the Polish Republic.
Assuming Great Britain and France cared more about their national honor that realpolitick, what would have occurred if they declared war on both the Soviet Union and Nazi Germany in September of 1939?