Military deception
Western front: In March 1917, leaders of the German Army on the Western Front decided to withdraw from their positions in France to the Hindenburg Line, a 90-mile long defensive position that ran from Arras to Laffaux. With Germany unable to conduct an offensive because of personnel losses earlier in the war, commanders intended for unrestricted submarine warfare and strategic bombing to weaken the British and French, giving the German army time to recuperate. In addition, the move to the Hindenburg Line supported the plan of commanders Paul von Hindenburg and Erich Ludendorff to shift the German focus to Russia and the Eastern Front. The withdrawal to the new defensive line would shorten Germany's front by 25 miles, enabling 13 divisions to be employed against the Russians.
Under the plan code named Operation Alberich, the Germans abandoned their old positions in a staged series that began in late February. The majority of their movement occurred between 16 and 21 March, and the full German withdrawal to the Hindenburg Line was completed on 5 April. The German withdrawal included numerous efforts to deceive the Allies, among them night movements and skeleton crews who remained behind to provide screening fire from machine guns, rifles, and mortars. The deception activities proved generally successful, and Germany completed its withdrawal to the Hindenburg Line largely unmolested.
In August 1918, the Allies intended to launch two offensives, one led by British Field Marshal Sir Douglas Haig, Commander-in-chief (C-in-C) of the British Expeditionary Force (BEF), near Amiens, and the other by American General John J. Pershing, C-in-C of the American Expeditionary Forces (AEF), near Saint-Mihiel. Prior to the Battle of Amiens, Haig's subordinate, General Henry Rawlinson, commander of the British Fourth Army, employed several deceptive tactics, including periods of radio silence by units involved in the coming attack and false radio traffic from other parts of the British lines. In addition, Rawlinson delayed troop movement for as long as possible prior to the start of the attack to prevent German observers from obtaining data on his disposition of forces, and moved troops and materiel almost entirely at night. The British offensive was immediately successful because they had maintained the element of surprise. British troops and tanks advanced eight miles, captured 400 artillery pieces, and inflicted 27,000 casualties, including 12,000 prisoners.
In an effort to gain an advantage near Saint-Mihiel, U.S. planners including Arthur L. Conger attempted to deceive the Germans into believing the American attack would come at Belfort, 180 miles to the south. False orders left where spies or informants could find them and staff officer reconnaissance activity created the appearance that the U.S. intended to conduct operations in and around Belfort. Pershing gained surprise at Saint-Mihiel and his offensive was successful. In 1926, Conger discovered from a former German officer that while the Belfort Ruse had not been completely successful, it had aided Pershing. Concerned that an Allied attack in the Belfort area was at least a possibility, many German units that could have reinforced German lines in the Saint-Mihiel area delayed movement from their rear area positions near Saint-Mihiel until it was too late. As a result of the success of Pershing's offensive, they did not have time to execute their withdrawal plans, and either abandoned their weapons and fled or were taken as prisoners.

Eastern front: At a war council held with senior commanders and the czar in April 1916, Russian General Aleksei Brusilov presented a plan to the Stavka (the Russian high command), proposing a massive offensive by his Southwestern Front against the Austro-Hungarian forces in Galicia. Brusilov's plan aimed to take some of the pressure off French and British armies in France and the Italian Army along the Isonzo Front and if possible to knock Austria-Hungary out of the war. As the Austrian army was heavily engaged in Italy, the Russian army enjoyed a significant numerical advantage in the Galician sector.
Brusilov's plan was approved, and he massed 40 infantry divisions and 15 cavalry divisions in preparation for the Brusilov Offensive. Deception efforts included false radio traffic, false orders sent by messengers who were intended to be captured, and equipment displays including dummy artillery to mislead Austria-Hungary as to the location of his units. Beginning in June 1916, Brusilov's surprise offensive caused Germany to halt its Western front attack on Verdun and transfer considerable forces to the Eastern front. It also severely reduced the fighting effectiveness of Austria-Hungary's forces, effectively depriving Germany of its most important ally.

Palestine: In October 1917, Edmund Allenby, commander of Britain's Egyptian Expeditionary Force, planned to attack the Ottomans in southern Palestine. Rather than repeat previous frontal assaults at Gaza, which had been unsuccessful, he planned a flanking attack at Beersheba. As part of the larger deception effort to convince the Ottomans that Gaza was his objective, an officer under Allenby's command executed a deceptive tactic now known as the Haversack Ruse. In this effort, usually attributed to Richard Meinertzhagen, the officer intentionally dropped a knapsack which contained false plans for an attack on Gaza, which the Ottomans recovered. As a result of the Haversack Ruse and other deceptive measures, the British surprised the Ottomans and achieved victory at the 31 October 1917 Battle of Beersheba.
Allenby again resorted to deception as he prepared to attack the Ottomans again at Tel Megiddo in September 1918. In preparation for the battle, British forces concealed the movement of three cavalry and several infantry divisions from the eastern end of the front line, the Jordan Valley, to the western end on the Mediterranean Sea. The single mounted division that remained in the east, reinforced with infantry, maintained the illusion that the Jordan Valley remained fully garrisoned. Deceptive measures included marching infantry into the valley during the day when they could be observed by the Ottomans, transporting them out by truck at night, then marching them back the next day. The tents of the departed units were left standing, and dummy horses, mules, and soldiers made from canvas and straw were displayed throughout the encampment. In addition, mules dragged branches up and down the valley to generate thick clouds of dust, giving the impression of more animals and men on hand than there actually were. Though the deceptions did not induce Otto Liman von Sanders, the commander of the Ottoman Army, to concentrate his forces on the eastern flank, as Allenby hoped, the Ottomans could not be certain of his intentions, so they could not mass their forces. With the Ottomans spread throughout their line, Allenby's forces had a numerical advantage at the 19 to 25 September Battle of Megiddo (1918), and attained victory over the Ottomans.

At sea: Britain's Royal Navy made extensive use of Q-ships to combat German submarines. Camouflaged to look like a civilian sailing vessel or decrepit tramp steamer, the Q-ships were decoys that carried concealed heavy guns. The function of the Q-ships was to appear to be an undefended target. If successful, German submarines would be lured to the surface to sink or destroy the ship by using deck guns, enabling the submarine to conserve its limited supply of torpedoes for use against warships. If a German U-boat surfaced, the Q-ship would immediately display the Royal Navy's White Ensign flag in compliance with international law, then deploy its guns against the submarine. In 150 engagements, British Q-ships destroyed 14 U-boats and damaged 60. 27 of the 200 Q-ships the British employed were lost to German attacks.

World War II: 

Eastern Front: Before Operation Barbarossa in June 1941, the German High Command explained away the creation of a massive force arrayed to invade the Soviet Union by informing Soviet leaders including Joseph Stalin that it was training in preparation for an invasion of the United Kingdom. The deception worked, and Stalin continued to ignore German preparations until after the invasion of the Soviet Union had actually commenced.
When the Allies on the Western Front planned the Normandy invasion for June 1944, the Soviet Union simultaneously planned for a major Eastern front offensive against German forces. Called Operation Bagration, this Soviet attack was designed to catch the Germans unprepared in Belorussia, at the center of their lines. To gain and maintain the element of surprise for Bagration, the Soviets executed a successful deception effort. The overall intent was to make German commanders believe the Soviets would only defend in the center of the front (Belorussia), while launching a major offensive to the south in Ukraine and Crimea and a feint to the north in Finland. Soviet military leaders successfully masked preparations for action in Belorussia and limited German reconnaissance in the center of the front while drawing German attention to deceptive activities in the north and south. Germany was caught off-guard at the start of the Bagration offensive on 23 June, and the Soviets quickly pushed retreating German forces from the Soviet Union all the way to Poland.

Western Front: 

Africa: From early 1941, Dudley Clarke commanded the 'A' Force, based in Cairo, Egypt, which developed much of the war's Allied deception strategy.