You might be interested in the responses this question has previously received.

Why didn't France and Britain declare war on the Soviet Union after they invaded Poland, Finland, Romania and the Baltic states in 1939 - 1940? submitted by doritosNachoCheese.

Here is the long and short of it, by /u/kieslowskifan:

The long and the short of it is that Anglo-French commitments to Poland only pertained to German aggression.[..]