LAURENCE REES: And so let’s just take these different theatres of war one by one. First, of course, there was Poland, where the war was over in a matter of weeks.

GEOFFREY WAWRO: Something about Polish lances charging German tanks. That’s not a recipe for success. But the Polish example is interesting in that it points to another reason why Blitzkrieg succeeded, which is often overlooked. In these early campaigns Hitler was fighting foes that were often distracted. So in the case of Poland they had to hive off a large part of their military to watch the Eastern border with the Soviet Union, which also marched in, so pressing the Poles from two sides. So the Poles could concentrate only a fraction of their strength against the Germans, they had to have a fraction watching the Russians and they were facing in two directions which made the German task all that much easier. So, again, it masked the fact that the German Army wasn’t sufficiently mechanised, its air power resources were prodigious by Polish standards, but certainly not enough to defeat France, Britain, the United States and Russia.

And why was there a phoney war after the invasion of Poland? Why was there this so-called 'Sitzkrieg' that went on from the Fall of 1939 until the Spring of 1940? Because the German Army was exhausted from its exertions in Poland. Most of its motor vehicles, tanks and trucks were out of commission and needed to be repaired, troops needed to be rested, everything had to be shifted back to the West. Hitler wanted an immediate invasion of the Low Countries to apply pressure to the French, to dissuade the British from involving themselves and to give better bases for the German Navy to operate against the UK if need be. The German Army said we’re not ready as everything was exhausted in the Polish campaign.