 :That's exactly why I wrote about complete non-importance paper treaties, declarations or pacts in this context. Britons (officially) saw the Soviet agression as they saw it simply because their were afraid of Hitler more than of Stalin, at this particular moment. If you want to have clearer view on how politics worked at the time, browse through the archives. Pacts were irrelevant, as they were frequently broken shortly after signing, with no sanctions at all. And official reaction of any politicians has exactly nothing to do with telling whether USSR was cobelligerent of Germany in 1939 or not - please read the definition.  