Poland was attacked on September 1, 1939. Why did Britain, France take two days to declare war on Germany?

.... so naïve were the prime minister and foreign secretary about the true nature of modern Blitzkrieg warfare that they genuinely imagined that there was even an outside possibility of Hitler simply calling off the attack.

Wehrmacht marches in Warsaw. Poland falls

Germany attacked Poland on September 1, 1939. Why did Britain and France take two days (They declared war on September 3) to do the same on Germany?

Andrew Roberts, the author of The Storm of war: The New History of the Second World War, gives the answers in an article in The Telegraph, UK.

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. Because they would not be providing any material help to Poland, indeed the British Army only started crossing over to the Continent after 3 September, there seemed to them to be no particular hurry to declare war, since doing nothing to help immediately struck them as little different from doing equally little a few days later. Moreover, the French Government of Edouard Daladier seemed to be dragging its feet, and both Governments believed a simultaneous declaration would have a far better effect.

Weak man at the helm: Chamberlain: Britain lost the chance to crush Germany

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. Although Chamberlain told the Commons that Britain 'would find it impossible to take part in a conference while Poland is being subjected to invasion, her towns are under bombardment and Danzig is being made the subject of a unilateral settlement by force', so naïve were the prime minister and foreign secretary about the true nature of modern Blitzkrieg warfare that they genuinely imagined that there was even an outside possibility of Hitler simply calling off the attack.

'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.'

To this day we do not know whether Mussolini made his offer of a peace conference in good faith or as a means of muddying the waters for the Western Allies just as his ally in the Pact of Steel, Adolf Hitler, concentrated on crushing the Poles, and anyhow it does not matter. The delay came to an end only when furious Cabinet ministers met behind the Speaker's Chair in the Commons and an outraged House imposed its will on Chamberlain and Halifax on the evening before war was finally declared. Appeasement was at an end, and not a moment too soon.