Why didn't England and France declare war on Russia as well as Germany on September 3rd, 1939?

Russia came in from Polands east and occupied it, so what was the reasoning to ignore Stalin?
Updates
1 y
Why didn't Stalin declare war on them when they war was declared on Germany seeing as how they had a non aggression pact together? Did Stalin believe he could occupy half of Poland while Germany was at war for taking the other half?
Why didn't England and France declare war on Russia as well as Germany on September 3rd, 1939?
Post Opinion