Following the end of the First World War, Poland was wedged uncomfortably between the two dominant nations of Germany and the Soviet Union. Poland was obliged to plot and negotiate to try and prevent them from realizing their ambitions to eviscerate the country.
As well as bitter ethnic battles between Germany and Poland for the political control of Upper Silesia, there were also the burning ambitions of Weimar Germany, and later Nazi Germany, to reclaim lands incorporated into the new state of Poland at Versailles.
Despite America's initial support, the US thereafter showed little interest in Poland's predicament. While France was a traditional friend to the Polish peoples, its political influence over eastern European affairs weakened.
The emergence of Hitler and the Nazis in Germany did little to bring the countries together. This drove them further apart as the Führer ramped up his rhetorical assault on the perceived injustices of Versailles.
London and Paris found themselves in the disagreeable position of seeing no option but to throw their whole weight behind the integrity of the Polish state if they were ever going to make any sort of stand against Nazi aggression.