Since England and France were treaty bound to protect Poland and declared war on Germany when it invaded Poland in 1939, why didn't they declare war on the USSR despite its simultaneous invasion of Poland?
The Polish asked the same question, but the Anglo-Polish military agreement contained secret clauses which restricted the obligations of Britain and France to war against Germany. In the case of invasion by a power that wasn't Germany, the agreement only required Britain and Poland to consult on coordinated efforts.