I'm reading James Holland's "War in the West", where he describes how the French very cautiously moved into Germany in September 1939, only to withdraw without meeting any serious resistance. Indeed he states that had they been more energetic, the way to Berlin would be open.
What were the reasons for this withdrawal, and why were they so tentative when the Germans were occupied with the invasion of Poland?
France did know its army wasn't ready, it's late. Government was expecting to build it up on summer 1940. It gave from around 1936 a deep reorganisation of its military industry (especially plane) and a massive increase of its defense budget.
France don't want a new war, still believes WW1 is "la der(nière) des der" (Last of the Last) and has no reason to fight in Europe. So, it spends very few for his troops. Until it s obvious Germany is a real threat.
Great Britain is in same shape but Navy is very powerful and is meant to hold a blocus and cripple german economy. It's a long term strategy, but could force Berlin to be defeated.
So, when Germany attacks Poland, french and british are not very confident and expect a long war. It's too soon to start the fight. Concerning USSR, Hitler has secured this front. So Wehrmacht may not be splitted for very long. Moreover, french officers (choosen by WW1 glorious leaders) expect WW2 to be a static war. France build la Ligne Maginot which "could hold", time to reinforce.
So, when Germany invades Poland, it gave several fights on its west border, but F+GB need to wait.
-> French source (concerning France rearmement), REVUE ECONOMIQUE, 1980, " Intervention étatique et réarmement en France 1935-1939" http://www.persee.fr/doc/reco_0035-2764_1980_num_31_4_408550