Declaring war on Russia wasn't the stupid mistake.
Going through Belgium was. That was the stated reason for Britain getting into the war. I would have dug in on the border with France and thrown the bulk of my forces at Russia before they could have mobilized. The French would not have had the "Gaul" to attack Germany. After beating Russia and securing the Baltic States, Poland, Byelorussia, and Ukraine, I would have pulled a GW and told France to, "Bring it on!" At which point they would have immediately soiled their pants and sued for peace.