Why did the allies declare war on Germany and not Russia following the 1939 invasion of Poland? They both invaded at the same time so I don't understand why Germany was singled out.
This question have been asked several times before, but I will attempt to answer it before.
First of all, no, they did not invade at the same time. The Germans invaded on the 1st of September 1939. The Soviet troops crossed the border on the 17th of September 1939.
The Soviets scrambled an ill-prepared invasion since the German invasion ws so successful that they feared they might not get their part if they did not act quickly. No-one had expected Poland to fall that quickly.
By the time the Soviets invaded, the Western Allies were at war with Germany (since the 3rd of September 1939) and had their hands full. They could not save Poland and did not want to add another enemy besides Germany to fight.