Neville Chamberlain? Churchill didn't take office until May of 1940. The non-aggression pact between Germany and Russia basically ended on June 22, 1941, when the Nazis invaded Russia. I don't know if Churchill has ever explained why England didn't declare war on Russia at the same time they did Germany. Perhaps it was because they were hoping to get the U.S. involved in the war in Europe before doing so. Maybe someone with more info knows the answer to it. This is just my own thoughts.
We only got involved because we got attacked at Pearl Habor.