Blitzkrieg
Blitzkrieg: from the German "lightning war", describing a battle strategy whereby rapid and unrestricted movement of troops and support allows no time for the opposition to set up a stable defense.
The strategy was developed as a reaction to 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. Breakouts were hard to achieve, attackers typically had to walk forward over contested (and cratered) ground, while defenders could rush troops to a new front line by rail at the nearest physical boundary. Another problem was that it was equally difficult to gather any sort of numerical advantage, as the troops attacked they were subject to artillery fire and thus unable to mass at a single point for the attack.
With the introduction of reliable tanks the attacking forces were no longer limited to the same sorts of speed as the defenders. The key then became to organize the troops into mobile forces with excellent communications and command, able to keep the momentum up while the battle unfolded. The basic concept was to concentrate all available forces at a single spot in front of the enemy lines, and then break a hole in it with artillery and infantry, easy enough to do even in WWI. Once the hole was opened the tanks would rush through and run hundreds of miles to the rear. This allowed the attacking force to fight against lightly armed logistics units, staving the enemy of information and supplies. In this way even a small force could destroy a much larger one through confusion, avoiding direct combat as much as possible.
The concept only became possible in the earlu 1930s, due to the increasing power and reliability of the internal combustion engine, as well as the invention of the portable radio which allowed for coordination of attacks. 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.
However it was put into practice only by the Germans, as Hitler felt that it was the only way for their much smaller forces to contest the overwhelming numbers of French and British forces. By the late 1930s they had re-organized their Army to include a number of elite Panzergruppen, divisions consisting almost entirely of tanks, infantry in half-track APC and trucks to supply them. To this they had added a new weapon, the dive bomber (specifically the Junkers Ju 87) to replace artillery and allow for "breakthrough" attacks even far behind the lines.
The theory was first put to use against Poland, where it proved effective although the mechanization of the troops at the time was limited. It demonstrated its true worth in 1940 against France, when a small force of panzers broke through the defensive lines and rushed to the coast before the defending forces could organize any sort of counterattack.
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 Operation Barbarossa campaign of 1941. Although the attack took huge areas of Russia, the overall strategic effect was more limited and the Red Army was able to regroup far to the rear, and eventually defeat the German forces several years later.
see also Military History