Why were Hitler's blitzkrieg tactics successful at the beginning of the war?
"Blitzkrieg" was a wedding of traditional concepts of maneuver, new tactics from the last year of the First World War, and the new technologies afforded by armour, radio, better vehicles and close air support.
- Best book: John English, "A Perspective on Infantry", an excellent survey on the history of tactics in the 20th Century.
The German Army thought hard about what the British and French did to them in 1918 (when even the toughest trench-lines were quickly penetrated) and drew the appropriate conclusions about tactics, training and organization. Success lay with far more than just new weapons like dive bombers and tanks.
Old ideas (going back to Napoleon) were combined with new training so that everyone from privates to Generals made decisions quickly and moved decisively. The idea was to keep the enemy off balance, and unable to react except to circumstances that had already changed. Instead of breaking his will by attrition and sheer destruction, the enemy's morale was the target -- to make him dazed and confused until he was trapped or cornered.
Radios made it easier to coordinate, and -- unlike in WW1 -- trucks and armour could move much faster than ever. What was more important were "mission tactics: (Auftragstaktik)... soldiers knew what their objectives were, but were free to work to achieve them using the best means at their disposal and the resources they were given.
Speed and surprise were vital. Soldiers were also supposed to improvise and adapt to any circumstances. Making a poor decision immediately was better than making an excellent one later. Tough problems were to be screened off and handled later, weak points were to be exploited immediately.
Besides the Germans in Poland, Norway and France, this is also how the British destroyed an Italian Army in Libya in two months in 1940-41. The Soviets had similar plans, but it took them two years to have officers and men capable of fighting this way.