Psychological warfare
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.
Grey propaganda (omissions, emphasis and racial/ethnic/religious bias): Largely truthful, containing no information that can be proven wrong; the source is not identified.
Black propaganda (commissions of falsification): Inherently deceitful, information given in the product is attributed to a source that was not responsible for its creation.
Lerner says grey and black operations ultimately have a heavy cost, in that the target population sooner or later recognizes them as propaganda and discredits the source. He writes, "This is one of the few dogmas advanced by Sykewarriors that is likely to endure as an axiom of propaganda: Credibility is a condition of persuasion. Before you can make a man do as you say, you must make him believe what you say.": 28  Consistent with this idea, the Allied strategy in World War II was predominantly one of truth (with certain exceptions).

In Propaganda: The Formation of Men's Attitudes, Jacques Ellul discusses psychological warfare as a common peace policy practice between nations as a form of indirect aggression. This type of propaganda drains the public opinion of an opposing regime by stripping away its power on public opinion. This form of aggression is hard to defend against because no international court of justice is capable of protecting against psychological aggression since it cannot be legally adjudicated. "Here the propagandists is [sic] dealing with a foreign adversary whose morale he seeks to destroy by psychological means so that the opponent begins to doubt the validity of his beliefs and actions."

Terrorism: According to Boaz Ganor, terrorism weakens the sense of security and disturbs daily life, damaging the target country's capability to function. Terrorism is a strategy that aims to influence public opinion into pressuring leaders to give in to the terrorists' demands, and the population becomes a tool to advance the political agenda.

By country: 

China: According to U.S. military analysts, attacking the enemy's mind is an important element of the People's Republic of China's military strategy. This type of warfare is rooted in the Chinese Stratagems outlined by Sun Tzu in The Art of War and Thirty-Six Stratagems. In its dealings with its rivals, China is expected to utilize Marxism to mobilize communist loyalists, as well as flex its economic and military muscle to persuade other nations to act in the Chinese government's interests. The Chinese government also tries to control the media to keep a tight hold on propaganda efforts for its people. The Chinese government also utilizes cognitive warfare against Taiwan.

France: The Centre interarmées des actions sur l'environnement is an organization made up of 300 soldiers whose mission is to assure to the four service arm of the French Armed Forces psychological warfare capacities. Deployed in particular to Mali and Afghanistan, its missions "consist in better explaining and accepting the action of French forces in operation with local actors and thus gaining their trust: direct aid to the populations, management of reconstruction sites, actions of communication of influence with the population, elites and local elected officials". The center has capacities for analysis, influence, expertise and instruction.

Germany: In the German Bundeswehr, the Zentrum Operative Kommunikation is responsible for PSYOP efforts. The center is subordinate to the Cyber and Information Domain Service branch alongside multiple IT and Electronic Warfare battalions and consists of around 1000 soldiers. One project of the German PSYOP forces is the radio station Stimme der Freiheit (Sada-e Azadi, Voice of Freedom), heard by thousands of Afghans. Another is the publication of various newspapers and magazines in Kosovo and Afghanistan, where German soldiers serve with NATO.

Iran: The Iranian government had an operation program to use the 2022 FIFA World Cup as a psyop against concurrent people's protests.

Israel: The Israeli government and its military make use of psychological warfare. In 2021, Israeli newspaper Haaretz revealed that "Abu Ali Express", a popular news page on Telegram and Twitter purportedly dedicated to "Arab affairs", was actually run by a Jewish Israeli paid consultant to the Israel Defense Forces (IDF). The IDF's psyops account had been the source of a number of noteworthy reports that were afterwards cited by the Israeli and international media.

Russia: 

Soviet Union: 

United Kingdom: The British were one of the first major military powers to use psychological warfare in the First and Second World Wars. In the current British Armed Forces, PsyOps are handled by the tri-service 15 Psychological Operations Group. (See also MI5 and Secret Intelligence Service). The Psychological Operations Group comprises over 150 personnel, approximately 75 from the regular Armed Services and 75 from the Reserves. The Group supports deployed commanders in the provision of psychological operations in operational and tactical environments.
The Group was established immediately after the 1991 Gulf War, has since grown significantly in size to meet operational requirements, and since 2015 has been one of the sub-units of the 77th Brigade, formerly called the Security Assistance Group.
In June 2015, NSA files published by Glenn Greenwald revealed details of the JTRIG group at British intelligence agency GCHQ covertly manipulating online communities. This is in line with JTRIG's goal: to "destroy, deny, degrade [and] disrupt" enemies by "discrediting" them, planting misinformation and shutting down their communications.
In March 2019, it emerged that the Defence Science and Technology Laboratory (DSTL) of the UK's Ministry of Defence (MoD) is tendering to arms companies and universities for £70M worth of assistance under a project to develop new methods of psychological warfare. The project is known as the human and social sciences research capability (HSSRC).

United States: The term psychological warfare is believed to have migrated from Germany to the United States in 1941. During World War II, the United States Joint Chiefs of Staff defined psychological warfare broadly, stating "Psychological warfare employs any weapon to influence the mind of the enemy. The weapons are psychological only in the effect they produce and not because of the weapons themselves." The U.S. Department of Defense (DoD) currently defines psychological warfare as:

"The planned use of propaganda and other psychological actions having the primary purpose of influencing the opinions, emotions, attitudes, and behavior of hostile foreign groups in such a way as to support the achievement of national objectives."
This definition indicates that a critical element of the U.S. psychological operations capabilities includes propaganda and by extension counterpropaganda. Joint Publication 3–53 establishes specific policy to use public affairs mediums to counter propaganda from foreign origins.
The purpose of United States psychological operations is to induce or reinforce attitudes and behaviors favorable to US objectives.  The Special Activities Center (SAC) is a division of the Central Intelligence Agency's Directorate of Operations, responsible for Covert Action and "Special Activities". These special activities include covert political influence (which includes psychological operations) and paramilitary operations.  SAC's political influence group is the only US unit allowed to conduct these operations covertly and is considered the primary unit in this area.
Dedicated psychological operations units exist in the United States Army and United States Marine Corps. The United States Navy and the 193rd Special Operations Wing of the United States Air Force also plans and executes limited PSYOP missions. United States PSYOP units and soldiers of all branches of the military are prohibited by law from targeting U.S. citizens with PSYOP within the borders of the United States (Executive Order S-1233, DOD Directive S-3321.1, and National Security Decision Directive 130). While United States Army PSYOP units may offer non-PSYOP support to domestic military missions, they can only target foreign audiences.
A U.S. Army field manual released in January 2013 states that "Inform and Influence Activities" are critical for describing, directing, and leading military operations. Several Army Division leadership staff are assigned to “planning, integration and synchronization of designated information-related capabilities."
In September 2022, the DoD launched an audit of covert information warfare after social media companies identified a suspected U.S. military operation.

See also: 

References: 

Bibliography: 

External links: 
Movie: Psywar: The Real Battlefield is the Mind by Metanoia films

Paul Myron Anthony Linebarger. Psychological Warfare at Project Gutenberg
The history of psychological warfare
IWS Psychological Operations (PsyOps) / Influence Operations Archived 8 December 2010 at the Wayback Machine
"Pentagon psychological warfare operation", USA Today, 15 December 2005
"U.S.