Wikipedia: Blitzkrieg

Blitzkrieg: from the German "lightning war", describing a battle strategy whereby rapid and unrestricted movement of troops and support is used to deliver fast blows against an enemy. The strategy was developed as a reaction against the stasis of World War I, when incredible casualties might be suffered in an offensive whose only result was to change the trench lines by a few hundred meters.

German strategic theorists devised the concept of blitzkrieg to take advantage of the increasing power and reliability of the internal combustion engine, though the idea was not theirs alone. A number of military figures in several nations realized that static warfare was an outmoded concept and could be defeated by concentrating forces on a narrow point in a fast thrust.

The blitzkrieg worked to its greatest effect against small nations or those with limited defensive capabilities. The fall of Poland in 1939 and the capitulation of France in 1940 are textbook examples of the successes that a blitzkrieg strategy can attain.

Blitzkrieg is not without its disadvantages; there is a real danger of the attacking force overextending its supply lines, and the strategy as a whole can be defeated by a determined foe who is willing to sacrifice territory for time in which to regroup and rearm, as was seen in the Barbarossa? campaign of 1941.