Who Started World War II?

[Joefour]

Originally Posted by Catfish

And on September 17th, 1939, 16 days after Nazi Germany invaded Poland from the west, the Soviet Union invaded Poland from the east.
The invasion and battle lasted for the following 20 days and ended on October 6th, 1939 with the two-way division and annexation of the entire territory of the Second Polish Republic by both Germany and the Soviet Union.

Then, England declared war to Germany. Because England did not "guarantee Poland's borders".
It only guaranteed Poland help in case of Germany violating Poland's borders.

So why didn't Britain declare war ALSO on the Soviet Union for invading Poland from the east? Instead, she allied herself with the russians. Makes no logical sense to me whatsoever, that is, unless something else was going on that we don't know about. Perhaps this should be explored too.