“Tuesday is the 70th anniversary of Germany's invasion of Poland. Leading historian Andrew Roberts explains the two-day delay behind Britain's declaration of war.
At 04.45am on Friday 1 September, German forces activated Plan White, which had been formulated that June by the German Army High Command, the Oberkommando des Heeres. On either side of a relatively weak and stationary centre, two powerful wings of the Wehrmacht would envelop Poland, crush its armed forces and capture Warsaw.
Because the British and French Governments had given a guarantee to Poland on 31 March 1939, with the British prime minister Neville Chamberlain formally promising her 'all support in the power' of the Allies should she be attacked, Hitler was forced to leave 40% of his 100-division army out in the West
Far from declaring war immediately on the morning of 1 September, it was not until 11am on Sunday 3 September, more than 48 hours after Blitzkrieg had been unleashed, that Neville Chamberlain finally announced to the British people, in his famous radio broadcast from No.10 Downing Street, that the Germans had not responded to his ultimatum 'and that consequently this country is at war with Germany.'
So what were the real reasons for the two-day delay in declaring war? It was true that Chamberlain and Halifax still hoped against hope (and rationality) that Hitler could be persuaded to withdraw from Poland once it was made clear to him that the Western Allies would stand by their guarantee.
Yet the central reason for the delay was an offer by Mussolini for an immediate Five Power conference of Britain, France, Poland, Germany and Italy.
'If the German Government should agree to withdraw their forces', Chamberlain stated with supreme wishful thinking, 'then His Majesty's Government would regard the position as being the same as it was before the German forces crossed the Polish frontier.'
Had a large, modern British Army and RAF been waiting up against the Siegfried Line in late August 1939, primed for action and superbly armed, trained, equipped and led, having been properly financed in the two decades since the Great War, and ranged alongside the French army to invade Nazi Germany the moment the Wehrmacht crossed the Polish border, history could have turned out very differently.”
http://www.telegraph.co.uk/news/newstopics/world-war-2/6105782/Second-World-War-Why-we-delayed-declaration-of-war.html