Psychological warfare
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. He became committed to the use of mass propaganda to influence the minds of the German population in the decades to come. By calling his movement The Third Reich, he was able to convince many civilians that his cause was not just a fad, but the way of their future. Joseph Goebbels was appointed as Propaganda Minister when Hitler came to power in 1933, and he portrayed Hitler as a messianic figure for the redemption of Germany. Hitler also coupled this with the resonating projections of his orations for effect.
Germany's Fall Grün plan of invasion of Czechoslovakia had a large part dealing with psychological warfare aimed both at the Czechoslovak civilians and government as well as, crucially, at Czechoslovakia's allies. It became successful to the point that Germany gained support of UK and France through appeasement to occupy Czechoslovakia without having to fight an all-out war, sustaining only minimum losses in covert war before the Munich Agreement.
At the start of the Second World War, the British set up the Political Warfare Executive to produce and distribute propaganda. Through the use of powerful transmitters, broadcasts could be made across Europe. Sefton Delmer managed a successful black propaganda campaign through several radio stations which were designed to be popular with German troops while at the same time introducing news material that would weaken their morale under a veneer of authenticity. British Prime Minister Winston Churchill made use of radio broadcasts for propaganda against the Germans. Churchill favoured deception; he said "In wartime, truth is so precious that she should always be attended by a bodyguard of lies.".

During World War II, the British made extensive use of deception – developing many new techniques and theories. The main protagonists at this time were 'A' Force, set up in 1940 under Dudley Clarke, and the London Controlling Section, chartered in 1942 under the control of John Bevan. Clarke pioneered many of the strategies of military deception. His ideas for combining fictional orders of battle, visual deception and double agents helped define Allied deception strategy during the war, for which he has been referred to as "the greatest British deceiver of WW2".
During the lead-up to the Allied invasion of Normandy, many new tactics in psychological warfare were devised. The plan for Operation Bodyguard set out a general strategy to mislead German high command as to the date and location of the invasion, which was obviously going to happen. Planning began in 1943 under the auspices of the London Controlling Section (LCS). A draft strategy, referred to as Plan Jael, was presented to Allied high command at the Tehran Conference. Operation Fortitude was intended to convince the Germans of a greater Allied military strength than was the case, through fictional field armies, faked operations to prepare the ground for invasion and "leaked" misinformation about the Allied order of battle and war plans.
Elaborate naval deceptions (Operations Glimmer, Taxable and Big Drum) were undertaken in the English Channel. Small ships and aircraft simulated invasion fleets lying off Pas de Calais, Cap d'Antifer and the western flank of the real invasion force. At the same time Operation Titanic involved the RAF dropping fake paratroopers to the east and west of the Normandy landings.

The deceptions were implemented with the use of double agents, radio traffic and visual deception. The British "Double Cross" anti-espionage operation had proven very successful from the outset of the war, and the LCS was able to use double agents to send back misleading information about Allied invasion plans. The use of visual deception, including mock tanks and other military hardware had been developed during the North Africa campaign. Mock hardware was created for Bodyguard; in particular, dummy landing craft were stockpiled to give the impression that the invasion would take place near Calais.
The Operation was a strategic success and the Normandy landings caught German defences unaware. Continuing deception, portraying the landings as a diversion from a forthcoming main invasion in the Calais region, led Hitler into delaying transferring forces from Calais to the real battleground for nearly seven weeks.

Vietnam War: The United States ran an extensive program of psychological warfare during the Vietnam War. The Phoenix Program had the dual aim of assassinating National Liberation Front of South Vietnam (NLF or Viet Cong) personnel and terrorizing any potential sympathizers or passive supporters. During the Phoenix Program, over 19,000 NLF supporters were killed. In Operation Wandering Soul, the United States also used tapes of distorted human sounds and played them during the night making the Vietnamese soldiers think that the dead were back for revenge.
The Vietcong and their forces also used a program of psychological warfare during this war. Trịnh Thị Ngọ, also known as Thu Hương and Hanoi Hannah, was a Vietnamese radio personality. She made English-language broadcasts for North Vietnam directed at United States troops. During the Vietnam War, Ngọ became famous among US soldiers for her propaganda broadcasts on Radio Hanoi. Her scripts were written by the North Vietnamese Army and were intended to frighten and shame the soldiers into leaving their posts. She made three broadcasts a day, reading a list of newly killed or imprisoned Americans, and playing popular US anti-war songs in an effort to incite feelings of nostalgia and homesickness, attempting to persuade US GIs that the US involvement in the Vietnam War was unjust and immoral. A typical broadcast began as follows:

How are you, GI Joe? It seems to me that most of you are poorly informed about the going of the war, to say nothing about a correct explanation of your presence over here. Nothing is more confused than to be ordered into a war to die or to be maimed for life without the faintest idea of what's going on.

21st century: The CIA made extensive use of Contra soldiers to destabilize the Sandinista government in Nicaragua. The CIA used psychological warfare techniques against the Panamanians by delivering unlicensed TV broadcasts. The United States government has used propaganda broadcasts against the Cuban government through TV Marti, based in Miami, Florida. However, the Cuban government has been successful at jamming the signal of TV Marti.
In the Iraq War, the United States used the shock and awe campaign to psychologically maim and break the will of the Iraqi Army to fight.
In cyberspace, social media has enabled the use of disinformation on a wide scale. Analysts have found evidence of doctored or misleading photographs spread by social media in the Syrian Civil War and 2014 Russian military intervention in Ukraine, possibly with state involvement. Military and governments have engaged in psychological operations (PSYOP) and informational warfare (IW) on social networking platforms to regulate foreign propaganda, which includes countries like the US, Russia, and China.
In 2022, Meta and the Stanford Internet Observatory found that over five years people associated with the U.S. military, who tried to conceal their identities, created fake accounts on social media systems including Balatarin, Facebook, Instagram, Odnoklassniki, Telegram, Twitter, VKontakte and YouTube in an influence operation in Central Asia and the Middle East. Their posts, primarily in Arabic, Farsi and Russian, criticized Iran, China and Russia and gave pro-Western narratives. Data suggested the activity was a series of covert campaigns rather than a single operation.
In operations in the South and East China Seas, both the United States and China have been engaged in "cognitive warfare", which involves displays of force, staged photographs and sharing disinformation. The start of the public use of "cognitive warfare" as a clear movement occurred in 2013 with China's political rhetoric.

Methods: Most modern uses of the term psychological warfare refer to the following military methods:

Demoralization:
Distributing pamphlets that encourage desertion or supply instructions on how to surrender.
Shock and awe military strategy.
Projecting repetitive and disturbing noises and music for long periods at high volume towards groups under siege like during Operation Nifty Package.
Propaganda radio stations, such as Lord Haw-Haw in World War II on the "Germany calling" station.
False flag events.
Terrorism
The threat of chemical weapons.
Information warfare.
Most of these techniques were developed during World War II or earlier, and have been used to some degree in every conflict since. Daniel Lerner was in the OSS (the predecessor to the American CIA) and in his book, attempts to analyze how effective the various strategies were. He concludes that there is little evidence that any of them were dramatically successful, except perhaps surrender instructions over loudspeakers when victory was imminent. Measuring the success or failure of psychological warfare is very hard, as the conditions are very far from being a controlled experiment.
Lerner also divides psychological warfare operations into three categories:

White propaganda (omissions and emphasis): Truthful and not strongly biased, where the source of information is acknowledged.