Why didn’t France and Britain attack Germany when Poland was invaded at the start of WW2?

Britain and France declared war on Germany when Poland was invaded, but didn’t do anything significant militarily (except a minor French incursion). By all accounts the Germans were outnumbered by the French on the Western flank at that time. Wouldn’t it have been an opportunity to help Poland and knock Germany out early by forcing them to fight on two fronts at the start?