False flag
Use in warfare: In land warfare, such operations are generally deemed acceptable under certain circumstances, such as to deceive enemies, provided the deception is not perfidious and that all such deceptions are discarded before opening fire upon the enemy. Similarly, in naval warfare such a deception is considered permissible, provided the false flag is lowered and the true flag raised before engaging in battle. Auxiliary cruisers operated in such a fashion in both World Wars, as did Q-ships, while merchant vessels were encouraged to use false flags for protection. Such masquerades promoted confusion not just of the enemy but of historical accounts. In 1914 the Battle of Trindade was fought between the British auxiliary cruiser RMS Carmania and the German auxiliary cruiser SMS Cap Trafalgar, which had been altered to look like Carmania. (Contrary to some accounts, the Carmania had not been altered to resemble the Cap Trafalgar.)
Another notable example was the World War II German commerce raider Kormoran, which surprised and sank the Australian light cruiser HMAS Sydney in 1941 while disguised as a Dutch merchant ship, causing the greatest loss of life on an Australian warship. While Kormoran was fatally damaged in the engagement and its crew captured, the outcome represented a considerable psychological victory for the Germans.
The British used a Kriegsmarine ensign in the St Nazaire Raid and captured a German code book. The old destroyer Campbeltown, which the British planned to sacrifice in the operation, was provided with cosmetic modifications that involved cutting the ship's funnels and chamfering the edges to resemble a German Type 23 torpedo boat. By this ruse the British were able to get within two miles (3 km) of the harbour before the defences responded, where the explosive-rigged Campbeltown and commandos successfully disabled or destroyed the key dock structures of the port.

Air warfare: Between December 1922 and February 1923, a commission of jurists at the Hague drafted a set of rules concerning the Control of Wireless Telegraphy in Time of War and Air Warfare. They included:

Art. 3. A military aircraft must carry an exterior mark indicating its nationality and its military character.
Art. 19. The use of false exterior marks is forbidden.
This draft was never adopted as a legally binding treaty, but the International Committee of the Red Cross states in its introduction on the draft: "To a great extent, [the draft rules] correspond to the customary rules and general principles underlying treaties on the law of war on land and at sea", and as such these two non-controversial articles were already part of customary law.

Land warfare: In land warfare, the use of a false flag is similar to that of naval warfare: the trial of Otto Skorzeny, who planned and commanded Operation Greif, by a U.S. military tribunal at the Dachau Trials included a finding that Skorzeny was not guilty of a crime by ordering his men into action in American uniforms. He had relayed to his men the warning of German legal experts: if they fought in American uniforms, they would be breaking the laws of war; however, they probably were not doing so simply by wearing the American uniforms. During the trial, a number of arguments were advanced to substantiate this position and the German and U.S. military seem to have been in agreement.
In the transcript of the trial, it is mentioned that Paragraph 43 of the Field Manual published by the War Department, United States Army, on 1 October 1940, under the entry Rules of Land Warfare states: "National flags, insignias and uniforms as a ruse – in practice it has been authorized to make use of these as a ruse. The foregoing rule (Article 23 of the Annex of the IV Hague Convention), does not prohibit such use, but does prohibit their improper use. It is certainly forbidden to make use of them during a combat. Before opening fire upon the enemy, they must be discarded."

As pretexts for war: 

Russo-Swedish War: In 1788, the head tailor at the Royal Swedish Opera received an order to sew a number of Russian military uniforms. These were then used by Swedes to stage an attack on Puumala, a Swedish outpost on the Russo-Swedish border, on 27 June 1788. This caused an outrage in Stockholm and impressed the Riksdag of the Estates, the Swedish national assembly, who until then had refused to agree to an offensive war against Russia. The Puumala incident allowed King Gustav III of Sweden, who lacked the constitutional authority to initiate unprovoked hostilities without the Estates' consent, to launch the Russo-Swedish War (1788–1790).

Franco-Prussian War: On July 13, 1870, Otto von Bismarck published the Ems Dispatch, an internal message from King Wilhelm I to Bismarck regarding certain demands made by the French ambassador. In the version purposefully released to the public, Bismarck instead made it sound like the King had gravely disrespected the ambassador – a ploy to trick Emperor Napoleon III into declaring war on the North German Confederation, with the end goal of unifying the northern and southern German states. This ploy would be successful, as Napoleon III would declare war six days later; and six months later, the Confederation would win and unify the German states.

Second Sino-Japanese War: In September 1931, Seishirō Itagaki and other Japanese mid- to junior-grade officers, without the knowledge of the Tokyo government, fabricated a pretext for invading Manchuria by blowing up a section of railway. Though the explosion was too weak to disrupt operations on the rail line, the Japanese nevertheless used the Mukden incident to seize Manchuria and create a puppet government in the form of the nominally independent state of Manchukuo.

World War II: 

Gleiwitz incident: The Gleiwitz incident in 1939 involved Reinhard Heydrich fabricating evidence of a Polish attack against Germany to mobilize German public opinion for war and to justify the war with Poland. Alfred Naujocks was a key organiser of the operation under orders from Heydrich. It led to the deaths of Nazi concentration camp victims who were dressed as German soldiers and then shot by the Gestapo to make it seem that they had been shot by Polish soldiers. This, along with other false flag operations in Operation Himmler, would be used to mobilize support from the German population for the start of World War II in Europe.
The operation failed to convince international public opinion of the German claims, and both Britain and France – Poland's allies – declared war two days after Germany invaded Poland.

Winter War: On 26 November 1939, the Soviet army shelled Mainila, a Russian village near the Finnish border. Soviet authorities blamed Finland for the attack and used the incident as a pretext to invade Finland, starting the Winter War, four days later.

Cuban Revolution: 

Operation Northwoods: Operation Northwoods, a 1962 plot proposed but never executed by the U.S. Department of Defense for a war with Cuba, involved scenarios such as fabricating the hijacking or shooting down of passenger and military planes, sinking a U.S. ship in the vicinity of Cuba, burning crops, sinking a boat filled with Cuban refugees, attacks by alleged Cuban infiltrators inside the United States, and harassment of U.S. aircraft and shipping and the destruction of aerial drones by aircraft disguised as Cuban MiGs. These actions would be blamed on Cuba, and would be a pretext for an invasion of Cuba and the overthrow of Fidel Castro's communist government. It was authorised by the Joint Chiefs of Staff, but then rejected by President John F. Kennedy. The surprise discovery of the documents relating to Operation Northwoods was a result of the comprehensive search for records related to the assassination of President John F. Kennedy by the Assassination Records Review Board in the mid-1990s. Information about Operation Northwoods was later publicized by James Bamford.

Russian invasion of Ukraine: In January and February 2022, Western government agencies predicted that Russia would use a false flag operation in Ukraine. In the days leading up to the Russian invasion of Ukraine on 24 February, the Russian government intensified its disinformation campaign, with Russian state media promoting false flags on a nearly hourly basis purporting to show Ukrainian forces attacking Russia, in a bid to justify an invasion of Ukraine. Many of the disinformation videos were poor and amateur in quality, with mismatching metadata showing incorrect dates, and evidence from Bellingcat researchers, and other independent journalists, showed that the claimed attacks, explosions, and evacuations in Donbas were staged by Russia.

As a tactic to undermine political opponents: 

Operation TPAJAX: On 4 April 1953, the CIA was ordered to undermine the government of Iran over a four-month period, as a precursor to overthrowing Prime Minister Mohammad Mosaddegh. One tactic used to undermine Mosaddegh was to carry out false flag attacks "on mosques and key public figures", to be blamed on Iranian communists loyal to the government.
The CIA operation was code-named TPAJAX. The tactic of a "directed campaign of bombings by Iranians posing as members of the Communist party", involved the bombing of "at least one" well known Muslim's house by CIA agents posing as Communists. The CIA determined that the tactic of false flag attacks added to the "positive outcome" of TPAJAX.
However, as "the C.I.A.