> They also allied with them

As your own link states, this wasn't an alliance. Also if you read the "background" section of the article, you'll see that they very much saw Nazi Germany has a major threat and wanted to contains them early by allying France and Britain.

But France and particularly Britain instead prefered trying to appease Germany until the last minute with the results we know.

The non-agression pact might seem cynical from the west point of view, but if you put yourself in the soviet shoes for a second, it was a logical move.

As for your second link it's really not very assertive, most of the claims made early after the war were rebutted by historians later on.