Specifically, how successful was the Blitzkrieg?

Blitzkrieg, meaning 'Lightning War', was the method of offensive warfare responsible for Nazi Germany's military successes in the early years of the Second World War. These techniques were used to great effect in 1939, when the Polish Army was destroyed in a series of encirclement battles. In May 1940 Hitler attacked France, his panzer divisions smashing through slow-moving French formations and cutting off the British Expeditionary Force at Dunkirk. Spectacular success was also achieved during the invasion of the Soviet Union in 1941 and large numbers of Soviet troops were captured. But Blitzkrieg was less successful against well organised defences. The flanks of rapidly advancing mobile forces were vulnerable to counter-attack. Soviet commanders learned to blunt German assaults with successive defence lines of guns and infantry. By 1943 the days of Blitzkrieg were over, and Germany was forced into a defensive war on all fronts.

The Blitzkrieg was so effective because it was "designed to create disorganization among enemy forces through the use of mobile forces and locally concentrated firepower."This allowed the Germans to have the upper hand when attacking and often was the reason for their success.