Fear and anxiety have been crucial elements in any general's strategy since the dawn of time. The Assyrian empire was reputedly formed through the use of generous amounts of terror and brutality. The Spartans struck such terror in their enemies that their forces often won their wars with other Greek city states simply by arriving on the battlefield. Gaius Marius, a Senator of the ancient Roman Republic, once quelled the threat of rebellion in a client kingdom by imposing upon his rival the might of a single Roman legion. The Mongol hordes claimed entire kingdoms as they swept through Europe through the use of scare tactics. The fact is, even before scorched earth strategies, fear and anxiety have long been in the arsenal of military commanders.
Fear and anxiety are immensely helpful in keeping enemy forces from not only fighting effectively, but in some cases, it might inspire them to leave your territory entirely. This was the case when Vlad III Dracula (yes, the name is in that format), more commonly referred to in history as Vlad the Impaler, faced against an overwhelming Turkish and Ottoman force. According to historical accounts, when the Shah-in Shah's forces encountered the massive field of impaled soldiers captured from previous encounters with Vlad's army, he turned back. He was greeted with an image that inspired fear and anxiety even in his supposedly invincible army: the sight of thousands of impaled Turkish and Ottoman troops. Most military historians acknowledge Vlad's tactic as one of the most effective uses of psychological warfare and terror in history.
The use of fear and anxiety, however, is not merely limited to enemy forces. During the Second World War, in the infamous Battle of Stalingrad, the Soviets employed fear and anxiety on their own forces. Soviet conscripts troops, forcibly taken for their homes and barely even armed properly for combat, were informed that to retreat would mean death. Faced with a choice of either German bullets or Russian bullets, the Red Army was reported to have charged repeatedly into German lines, seemingly showing little regard for their own safety.
History also has several examples of fear and anxiety having effects on the maneuvering of troops. One example occurred again during World War II. German troops of the time were notoriously effective, such that the Allied commanders felt it was a tactical mistake to send their troops to face Hitler's blitzkrieg army. The front lines also experienced fear and anxiety, particularly when faced with sizable German infantry and panzer divisions. As a remedy, American and British commanders took a cue from the Soviet Red Army on the Eastern front and ordered their forces to engage the less well-trained Romanian and Italian armies, which lacked the discipline and equipment of their German counterparts.
 
The tank was initially developed as a weapon of intimidation, designed to scare infantry into breaking rank as the approaching mass of steel and gunfire came closer to them. The tanks were maneuvered straight into enemy infantry lines, regardless of how much damage the machines would have taken, simply to use the tank's intimidating effect to the fullest.
The fact is, scare tactics were, are, and will always be part of warfare for as long as there are human beings fighting on the front lines. Terrorizing your own troops as a substitute for morale and horrifying your opponents as a means of demoralizing them will inevitably be considered part and parcel of any comprehensive guide to warfare.