Why did Great Britain and France sit on their hands after declaring war on Nazi Germany following Hitler's invasion of Poland?

The major
reason why Great Britain and France did not do anything was that there was very little that they
could do.  When Germany attacked Poland, there was no plausible way for the Allies to actually
do anything to help that country. 

There were two reasons for this.  One was
military and the other was geographical.  On the military side, the French and English were
simply not...