Excerpts from Books and Wikipedia
"On September 17, in accordance with a secret agreement and the Nazi-Soviet pact, the Soviet army invaded Poland from the east. A German-Russian border was established within former Polish territory, and Poland ceased to exist as an independent country. On September 3, Britain and France declared war on Germany in accord with the guarantees they had given to Poland, but failed to take any immediate military action to help the Polish people." [Furtado: 1001 Days]