Defensive Tactics
Scorched earth is a military tactic which involves destroying anything that might be useful to the enemy whilst withdrawing from an area. The name refers to the practice of burning crops to deny the enemy food sources. The practice may be carried out by an army in enemy territory, or by an army in its own home territory. An example of the latter occurred in World War II when the Soviet Red Army salted their own lands as the Nazis forced them to retreat back through it, preventing the Nazis from growing crops on it. This example also illustrates another important aspect to consider about scorched earth tactics: when the Soviets reclaimed the territory, they were equally unable to use it. In warfare, a booby trap is an antipersonnel device, such as a landmine or grenade, placed in building or in a noncombat area that has a psychological draw for enemy soldiers. A booby trap is distinguished from a land mine by the fact that it is an improvised weapon, perhaps made from an artillery shell, or a grenade, or a quantity of high explosives, whereas a land mine is manufactured for its specific purpose. A booby trap may be buried in the manner common with land mines, or not, though as a rule it is concealed in some fashion, and set to be detonated by means of pressure or a trip wire. During World War II retreating British and Canadian soldiers in France in 1940 made booby traps from artillery and mortar shells, burying them in roads behind them as they retreated, or hanging them from trees concealed by the leaves and rigged with tripwires concealed in the grass around the tree.

Sieges in Modern Warfare
Mainly as a result of the increasing firepower (such as machine guns) available to defensive forces, First World War trench warfare briefly revived a form of siege warfare. Although siege warfare had moved out from an urban setting because city walls had become ineffective against modern weapons, trench warfare was nonetheless able to utilize many of the techniques of siege warfare in its prosecution (sapping, mining, barrage and, of course, attrition) but on a much larger scale and on a greatly extended front. The development of the armored tank at the end of World War I swung the pendulum back in favor of maneuver. The Blitzkrieg of the Second World War showed, however, that fixed fortifications are easily defeated by maneuver instead of frontal assault or long sieges. Battles that would have taken weeks of siege could now be avoided with the careful application of air power (such as the German paratrooper capture of Fort Eban, Belgium, early in World War II). The most important sieges of the Second World War were on the Eastern Front where bloody urban warfare marked the battles of Leningrad and Stalingrad. In these battles, the ruins of an urban landscape proved to be as effective obstacles to an advancing army as any fortifications. The battles of Dien Bien Phu (1954) and Khe Sanh (1968) possessed siege-like characteristics. In both cases, the Vietcong were able to cut off the opposing army by capturing the surrounding rugged terrain. At Dien Bien Phu, the French were unable to use air power to overcome the siege and were defeated. But at Khe Sanh a mere 14 years later, advances in air power allowed the United States to overcome the siege and win the battle.