That part of Poland had traditionally been German territory. And it's also a little known and less discussed fact that ethnic Germans were being slaughtered by the Pols prior to Hitler's invasion. What interest could Britain and France possibly have in protecting Poland? If that were their true motives for declaring war, they'd have also declared war on the Soviet Union for invading Poland. And speaking of framing, it was the Soviets who decapitated what was left of the Polish leadership and tried to blame it on the Germans. The NKVD murdered 15,000 Polish officers, politicians and intellectuals and buried them in mass graves.