Psychological warfare
History: 

Early: Since prehistoric times, warlords and chiefs have recognized the importance of weakening the morale of their opponents. According to Polyaenus, in the Battle of Pelusium (525 BC) between the Persian Empire and ancient Egypt, the Persian forces used cats and other animals as a psychological tactic against the Egyptians, who avoided harming cats due to religious belief and superstitions.
Currying favor with supporters was the other side of psychological warfare, and an early practitioner of this was Alexander the Great, who successfully conquered large parts of Europe and the Middle East and held on to his territorial gains by co-opting local elites into the Greek administration and culture. Alexander left some of his men behind in each conquered city to introduce Greek culture and oppress dissident views. His soldiers were paid dowries to marry locals in an effort to encourage assimilation.
Genghis Khan, leader of the Mongolian Empire in the 13th century AD employed less subtle techniques. Defeating the will of the enemy before having to attack and reaching a consented settlement was preferable to facing his wrath. The Mongol generals demanded submission to the Khan and threatened the initially captured villages with complete destruction if they refused to surrender. If they had to fight to take the settlement, the Mongol generals fulfilled their threats and massacred the survivors. Tales of the encroaching horde spread to the next villages and created an aura of insecurity that undermined the possibility of future resistance.
Genghis Khan also employed tactics that made his numbers seem greater than they actually were. During night operations he ordered each soldier to light three torches at dusk to give the illusion of an overwhelming army and deceive and intimidate enemy scouts. He also sometimes had objects tied to the tails of his horses, so that riding on open and dry fields raised a cloud of dust that gave the enemy the impression of great numbers. His soldiers used arrows specially notched to whistle as they flew through the air, creating a terrifying noise.
Another tactic favored by the Mongols was catapulting severed human heads over city walls to frighten the inhabitants and spread disease in the besieged city's closed confines. This was especially used by the later Turko-Mongol chieftain.
The Muslim caliph Omar, in his battles against the Byzantine Empire, sent small reinforcements in the form of a continuous stream, giving the impression that a large force would accumulate eventually if not swiftly dealt with.
During the early Qin dynasty and late Eastern Zhou dynasty in 1st century AD China, the Empty Fort Strategy was used to trick the enemy into believing that an empty location was an ambush, in order to prevent them from attacking it using reverse psychology. This tactic also relied on luck, should the enemy believe that the location is a threat to them.
In the 6th century BCE Greek Bias of Priene successfully resisted the Lydian king Alyattes by fattening up a pair of mules and driving them out of the besieged city. When Alyattes' envoy was then sent to Priene, Bias had piles of sand covered with wheat to give the impression of plentiful resources.
This ruse appears to have been well known in medieval Europe: defenders in castles or towns under siege would throw food from the walls to show besiegers that provisions were plentiful. A famous example occurs in the 8th-century legend of Lady Carcas, who supposedly persuaded the Franks to abandon a five-year siege by this means and gave her name to Carcassonne as a result.
During the Granada War, Spanish captain Hernán Pérez del Pulgar routinely employed psychological tactics as part of his guerrilla actions against the Emirate of Granada. In 1490, infiltrating the city by night with a small retinue of soldiers, he nailed a letter of challenge on the main mosque and set fire to the alcaicería before withdrawing.
In 1574, having been informed about the pirate attacks previous to the Battle of Manila, Spanish captain Juan de Salcedo had his relief force return to the city by night while playing marching music and carrying torches in loose formations, so they would appear to be a much larger army to any nearby enemy. They reached the city unopposed.
During the Attack on Marstrand in 1719, Peter Tordenskjold carried out military deception against the Swedes.  Although probably apocryphal, he apparently succeeded in making his small force appear larger and feed disinformation to his opponents, similar to the Operations Fortitude and Titanic in World War II.

World War I: The start of modern psychological operations in war is generally dated to World War I. By that point, Western societies were increasingly educated and urbanized, and mass media was available in the form of large circulation newspapers and posters. It was also possible to transmit propaganda to the enemy via the use of airborne leaflets or through explosive delivery systems like modified artillery or mortar rounds.
At the start of the war, the belligerents, especially the British and Germans, began distributing propaganda, both domestically and on the Western front. The British had several advantages that allowed them to succeed in the battle for world opinion; they had one of the world's most reputable news systems, with much experience in international and cross-cultural communication, and they controlled much of the undersea communications cable system then in operation. These capabilities were easily transitioned to the task of warfare.
The British also had a diplomatic service that maintained good relations with many nations around the world, in contrast to the reputation of the German services. While German attempts to foment revolution in parts of the British Empire, such as Ireland and India, were ineffective, extensive experience in the Middle East allowed the British to successfully induce the Arabs to revolt against the Ottoman Empire.
In August 1914, David Lloyd George appointed a Member of Parliament (MP), Charles Masterman, to head a Propaganda Agency at Wellington House. A distinguished body of literary talent was enlisted for the task, with its members including Arthur Conan Doyle, Ford Madox Ford, G. K. Chesterton, Thomas Hardy, Rudyard Kipling and H. G. Wells. Over 1,160 pamphlets were published during the war and distributed to neutral countries, and eventually, to Germany. One of the first significant publications, the Report on Alleged German Outrages of 1915, had a great effect on general opinion across the world. The pamphlet documented atrocities, both actual and alleged, committed by the German army against Belgian civilians. A Dutch illustrator, Louis Raemaekers, provided the highly emotional drawings which appeared in the pamphlet.
In 1917, the bureau was subsumed into the new Department of Information and branched out into telegraph communications, radio, newspapers, magazines and the cinema. In 1918, Viscount Northcliffe was appointed Director of Propaganda in Enemy Countries. The department was split between propaganda against Germany organized by H.G Wells, and propaganda against the Austro-Hungarian Empire supervised by Wickham Steed and Robert William Seton-Watson; the attempts of the latter focused on the lack of ethnic cohesion in the Empire and stoked the grievances of minorities such as the Croats and Slovenes. It had a significant effect on the final collapse of the Austro-Hungarian Army at the Battle of Vittorio Veneto.
Aerial leaflets were dropped over German trenches containing postcards from prisoners of war detailing their humane conditions, surrender notices and general propaganda against the Kaiser and the German generals. By the end of the war, MI7b had distributed almost 26 million leaflets. The Germans began shooting the leaflet-dropping pilots, prompting the British to develop unmanned leaflet balloons that drifted across no-man's land. At least one in seven of these leaflets were not handed in by the soldiers to their superiors, despite severe penalties for that offence. Even General Hindenburg admitted that "Unsuspectingly, many thousands consumed the poison", and POWs admitted to being disillusioned by the propaganda leaflets that depicted the use of German troops as mere cannon fodder. In 1915, the British began airdropping a regular leaflet newspaper Le Courrier de l'Air for civilians in German-occupied France and Belgium.
At the start of the war, the French government took control of the media to suppress negative coverage. Only in 1916, with the establishment of the Maison de la Presse, did they begin to use similar tactics for the purpose of psychological warfare. One of its sections was the "Service de la Propagande aérienne" (Aerial Propaganda Service), headed by Professor Tonnelat and Jean-Jacques Waltz, an Alsatian artist code-named "Hansi". The French tended to distribute leaflets of images only, although the full publication of US President Woodrow Wilson's Fourteen Points, which had been heavily edited in the German newspapers, was distributed via airborne leaflets by the French.
The Central Powers were slow to use these techniques; however, at the start of the war the Germans succeeded in inducing the Sultan of the Ottoman Empire to declare 'holy war', or Jihad, against the Western infidels. They also attempted to foment rebellion against the British Empire in places as far afield as Ireland, Afghanistan, and India. The Germans' greatest success was in giving the Russian revolutionary, Lenin, free transit on a sealed train from Switzerland to Finland after the overthrow of the Tsar. This soon paid off when the Bolshevik Revolution took Russia out of the war.

World War II: Adolf Hitler was greatly influenced by the psychological tactics of warfare the British had employed during World War I, and attributed the defeat of Germany to the effects this propaganda had on the soldiers.