This is one of several nodes dedicated to the code names of World War II. I hope to expand it over time, so please let me know of others, provided they have good sources.

This list will include all code names officially used by the U.S. Army, even if the code names described action by other allied armies, or other branches of service in the U.S.

ABC-1
Agreements reached at Washington Conference, January-March 1941.
ABDA
acronym for the 1942 cooperative American-British-Dutch-Australian defense of the Pacific and Indian Ocean theater
ABERDEEN
code name for Chindit stronghold near Manhton, Burma
ACCOLADE
seizure of the Dodecanese
ALABASTER
Troop convoy from Greenock to Iceland 17 May 1940.
ALAMO
code name for the task force (built around Headquarters, Sixth Army) operating directly under General Headquarters, Southwest Pacific Area for operations in New Guinea
ALPHA
plan to defend Kunming and Chungking
ALSOS
code name for the Allied special intelligence forces' mission to collect information about German nuclear fission developments
ANAKIM
Plan for recapture of Burma.
ANCHORAGE
Code name originally used for projected Hansa Bay operation. ANCHORAGE was used for ALAMO Force journals kept during the Admiralties operation.
ANVIL
The planned 1944 Allied invasion of southern France in the Toulon-Marseille area--see later DRAGOON.
ARCADIA
U.S.-British Conference held in Washington, December 1941-January 1942.
ARGONAUT
International Conference held at Malta and Yalta, January-February 1945.
ARGUMENT
plan for USSTAF air operations against German aircraft factories, February 1944.
AVALANCHE
(9 September 1943) invasion of Italy at Salerno
AXIOM
code name for the mission sent to London in February 1944 to urge CULVERIN
BACKHANDER
code name for the task force to conduct operations on Cape Gloucester on the island of New Britain
BARCLAY
plan to induce the Axis to give priority to maintaining and reinforcing its forces in southern France and the Balkans
BARRACUDA
plan (cancelled) for an airborne and amphibious assault on Naples
BARRISTER
Plan for capture of Dakar (formerly BLACK and PICADOR).
BAYTOWN
(3 September 1943) British invasion of Italy at Reggio di Calabria mounted from Sicily
BAZAAR
Plan for American air support of USSR In event of Japanese attack on Soviet Union. Also code name for U. S. survey project of air facilities in Siberia.
BETA
plan to open a port on the coast of China
BIRCH
Christmas Island.
BLACK
Plan for seizure of Dakar (later PICADOR and BARRISTER).
BLACKCOCK
British 12 Corps operation to clear the German salient between the Meuse and Roer-Wurm Rivers from Roermond southward
BLACKPOOL
code name for Chindit roadblock on the railroad near Namkwin, Burma
BLEACHER
Tongatabu.
BLOCKBUSTER
plan for Canadian II Corps offensive in the Calcar-Udem-Xanten area
BOBCAT
Borabora.
BOLERO
(1942) plan for offensive operations against Germany on the European mainland and for the build-up of US forces and supplies in the United Kingdom for the cross-Channel attack
BOSTON
Southwest Pacific Area of Operations plan to occupy and construct an airfield in the Abau-Mullins Harbor region of New Guinea
BRASSARD
plan for operations against the island of Elba
BREASTPLATE
plan (cancelled) for a seaborne attack from Malta against Sousse
BREWER
Operations in the Admiralties.
BRIMSTONE
plan (canceled) for the capture of Sardinia
BROADWAY
code name for a drop site about fifty miles northwest of Indaw, Burma, used to resupply the Chindits
BUCCANEER
plan (cancelled) for the amphibious capture of the Andaman Islands
BUFFALO
operation to break out of the Anzio beachhead
BUTTRESS
British operation (cancelled) against the toe of the Italian peninsula involving landings at Goija staged from North Africa
BYPRODUCT
code name for Trobriand Islands (not an actual operation)
CACTUS
operation to capture Guadalcanal (a sub-task of WATCHTOWER)
CAMEL
code name for the task force for the invasion of southern France built around the 36th Infantry Division
CAPITAL
plan for the attack across the Chindwin River to Mandalay
CAPRI
plan for an attack against Médenine
CARBONADO
plan to open a port on the coast of China (revised BETA)
CARPETBAGGER
operation to airdrop supplies from the United Kingdom to patriot forces in western Europe
CARTWHEEL
operations to capture the Solomon Islands, New Britain, and New Ireland involving convergent operations from the South Pacific and Southwest Pacific Areas of Operations targeted against Rabaul
CASANOVA
diversionary operation by the 95th Infantry Division during operations against Metz
CATCHPOLE
plan for operations against the Eniwetok and Ujelang Atolls in the Marshall Islands
CATHERINE
Plan for sending British naval forces into the Baltic Sea to gain control of those waters and stop Swedish ore traffic.
CAUSEWAY
operation plan (August 1944) to capture Formosa which was not executed
CENT
code name for the beaches at Scoglitti, Sicily
CHAMPION
Late 1943 plan for general offensive in Burma.
CHATTANOOGA CHOO CHOO
AEAF operations against German train movements in France and Germany
CHESTNUT
code name for four airborne missions sent by Montgomery in an effort to reinforce his army in Sicily with airborne troops
CHRONICLE
operation to capture Woodlark Island and Kiriwina
CLEANSLATE
operation to occupy the Russell Islands
CLIPPER
British 30 Corps offensive to reduce the Geilenkirchen salient
COBRA
plan for the First Army's operation designed to penetrate the German defenses west of St. Lô and secure Coutances, France
COMET
British plan, not carried out, for an air drop on 7 September 1944 in the Arnhem-Nijmegen area.
CORKSCREW
(June 1943) conquest of the island of Pantelleria
CORONET
operation (not executed) to invade the island of Honshu at the Tokyo plain. Its launch date of 1 December 1945 was rendered unnecessary by the dropping of the atomic bombs in August.
COTTAGE
(1943) recapture of Kiska
CRICKET
code name for the Malta portion of the ARGONAUT Conference
CROSSBOW
operations by the Royal Air Force against German V-weapon experimental bases
CROSSWORD
code name for a covert operation intended to lead to the surrender of Germany (2 May 1944)
CUDGEL
plan (cancelled) for small-scale operations on the Arakan coast of Burma
CULVERIN
Plan for assault on Sumatra.
CYCLONE
operation to capture Noemfoor (1944)
DELTA
code name for the task force for the invasion of southern France built around the 45th Infantry Division
DEXTERITY
operations in western New Britain and Saidor
DIADEM
(11 May 1944) Allied offensive in Italy
DIME
code name for the beaches at Gela, Sicily
DIRECTOR
code name for the task force for the invasion of Arawe in New Britain
DIXIE
code name for the US observer mission to the Chines communists
DOVETAIL
rehearsal in Fiji of the operation to capture Guadalcanal and a lodgement in the southern Solomon Islands (a sub-task of PESTILENCE)
DRACULA
(1944) plan for an attack on Rangoon
DRAGOON
final plan for the invasion of southern France (15 August 1944); code name was changed from ANVIL on 27 July 1944)
DRYGOODS
logistical operation to mass supplies at Guadalcanal in February 1943
ECLIPSE
name given to Operation TALISMAN after a presumed compromise of security
ELKTON
General Douglas MacArthur's plan (not executed) for the capture of Rabaul
ENCORE
(February 1945) IV Corps limited objective operation against Mounte Belvedere
END RUN
code name for the task force built around the survivors of Task Force GALAHAD used for the drive on Myitkyina, Burma
ENIGMA
see ULTRA
EUREKA
The Tehran Conference, 28 November-1 December 1943.
FANTAN
Fiji Islands.
FATIMA
code name for the Mason-MacFarlane Mission
FELIX
plan to capture Gibraltar and close the straits of Gibraltar
FIREBRAND
(1943) invasion of Corsica
FIVESOME
codeword to cover the multi-national agreement for the coordination of operations in the Southwest Pacific
FLASHPOINT
(March 1945) Ninth Army assault crossing of the Rhine as part of Operation PLUNDER
FLAX
air operations to disrupt the flow of German air transports from Italy to Sicily and Tunisia
FLINTLOCK
operation to capture the Marshall Islands
FORAGER
operation to capture the Mariana Islands
FOURTH TERM
(February 1945) IV Corps limited objective operation in the Serchio Valley
FRANTIC
Air Force shuttle bombing of Axis-controlled Europe from bases in the United Kingdom, Italy, and the USSR
FRY
operation to occupy four islands in Lake Comacchio, Italy
GALAHAD
code name for American long-range penetration groups in Burma
GALVANIC
operation to capture the Gilbert Islands and Makin
GANGWAY
plan (cancelled) for the unopposed landing at Naples
GARDEN
World War II--see MARKET
GIANT I
plan (canceled) for an air landing and airborne drop along the Volturno River
GIANT II
plan (canceled) for an airborne drop near Rome
GOBLET
plan (canceled) for the invasion of Italy at Cotrone
GOLD
code name for the assault beach at Normandy assigned to British 30 Corps
GOLDFLAKE
code name for the movement of Canadian I Corps from Italy to the European Theater of Operations
GRANITE
tentative plan (13 January 1944) by ADM Chester Nimitz for the capture of the Marshall Islands and other 1944 offensive operations
GRAY
Pre-War Plan for capture and occupation of the Azores.
GREEN
Prewar plan of operations in event of war with Mexico.
GRENADE
(February 1945) assault crossing of the Roer River by Ninth Army followed by a drive northeast to link with the First Canadian Army along the Rhine River; large-scale offensive by 21 Army Group from the Roer to the Rhine
GYMNAST
Early (1941) plan for invasion of North Africa, referring to either the American plan for landing at Casablanca or the British plan for landing farther eastward on the Mediterranean coast (see SUPER-GYMNAST).
HALPRO
Halverson Project -bombing detachment for China- Burma-India theater.
HARDIHOOD
code name for Allied aid to Turkey in an effort to induce that nation to enter the war
HOLLY
Canton Island.
HUDDLE
operation to occupy Ndeni (a sub-task of PESTILENCE)
HURRICANE
operation to capture Biak (1944)
HUSKY
Allied invasion of Sicily in July 1943.
ICEBURG
operation to capture Okinawa (1945)
INDEPENDENCE
French offensive towards Belfort, France
INDIGO
Pre-war Plan for movement of troops to Iceland.
INTERLUDE
code name for the rehearsal for the capture of Morotai
JUNO code
name for the Normandy assault beach assigned to the Canadian 3rd Division
JUPITER
Plan for operations in northern Norway.
KAPUT
(April 1945) Ninth Army assignment of defeating an incursion into the zone of the XIII Corps and of clearing a sector along the Elbe River
KING TWO
operation to capture Leyte and the southern Philippines (1944)
LEATHERBACK
cover name for Woodlark Island (not an operation name)
LEVER
operation in Italy to clear the area between Reno and the southwest shore of Lake Comacchio
LIGHTFOOT
British offensive operations in Libyan Desert, launched front El Alamein in October 1942.
LILLIPUT
operation to defend Buna (1942)
LONDON
phase line for XVIII Airborne Corps near Wesel, Germany
LUMBERJACK
(February-March 1945) converging offensive by First and Third Armies to create a pocket of trapped Germans in the Eifel
MAGIC
Code name applied to intercepted and decoded Japanese messages.
MAGNET
Movement of first U. S. forces to Northern Ireland.
MAGNETO
code name for the Yalta portion of the Argonaut Conference
MAILFIST
(1945) recapture of Singapore
MAINYARD
code name for the island of Guadalcanal (not an operation name)
MALLORY MAJOR
air offensive against the bridges over the Po River
MANNA
British occupation of southern Greece
MARKET-GARDEN
Operation in September 1944 to establish bridgeheads across three rivers in the Netherlands (Maas, Waal, and Lower Rhine) and reach the IJssel Meer. Airborne phase was called Operation MARKET. Companion ground attack was Operation GARDEN.
MARS
code name for the US task force in the China-Burma-India Theater of Operations that was the 5332d Brigade (Provisional)
MICHAELMAS
code name for the task force for the seizure of Saidor on New Guinea
MIKE FOUR
plan for the invasion of Nasugbu and Balayan Bays
MIKE ONE
plan for the invasion of Lingayen Gulf
MIKE SEVEN
plan for the invasion of the Zambales coast
MIKE SIX
plan for the invasion of Batangas and Tayabas Bays
MIKE THREE
plan for the invasion of Vigan
MIKE TWO
plan for the invasion of Dingalan Bay
MINCEMEAT
cover plan to divert attention from Operation HUSKY to induce the Axis to believe that Allied objectives were Sardinia and the Peloponnesus instead of Sicily
MINERVA
code name for the 6 November 1942 embarkation of General Henri Giraud from southern France
MODICUM
Party sent to London to present Marshall Memorandum, April 1942.
MONTCLAIR
World War II; final name given to PRINCETON
MUSKET
plan for a projected landing in 1943 on the heel of the Italian peninsula near Taranto
MUSKETEER
Code name for a series of plans covering operations for recapture of Philippine Islands.
MUSTANG plan
(canceled) for an overland seizure of Naples after initial landings in Calabria
NABOB
Northern Ireland.
NEPTUNE
Actual 1944 operations within OVERLORD. Used for security reasons after September 1943 on all OVERLORD planning papers that referred to target area and date.
NEW GALAHAD
code name for American long-range penetration groups in Burma
NEW YORK
XVIII Airborne Corps phase line in the Ringenberk-Krudenberg area in Germany
NUNTON
(March 1944) Allied cover and deception plan
OCTAGON
code name for the Quebec Conference, September 1944
OLIVE
(September 1944) attack on the Gothic Line in Italy
OLYMPIC
plan (not executed) for the March 1946 invasion of the island of Kyushu
OMAHA
code name for the assault beach at Normandy assigned to V Corps
ORANGE
Pre-war plan within the RAINBOW matrix for unilateral conflict between the United States and Japan; ORANGE 1 was approved in 1938, ORANGE 3 in April 1941
OVERLORD
(6 June 1944) Allied cross-Channel invasion of northwest Europe
PANTHER
British 10 Corps drive across the Garigliano River in Italy
PARIS
XVIII Airborne Corps phase line west of Erle, Germany
PERPETUAL
(11 November 1942) Eastern Task Force reserve's landing at Bougie
PERSECUTION
operation to capture Aitape (1944)
PESTILENCE
COMSOPAC OPLAN 1-42, 16 July 1942, plan for "Task One" to launch the initial offensive operation in Guadalcanal and the southern Solomon Islands
PICADILLY
code name for a drop site for the Chindits in Burma
PICADOR
Plan for Seizure of Dakar (formerly BLACK, later BARRISTER).
PIGSTICK
limited operation (cancelled) on the south Mayu Peninsula
PLOUGH
Project for training U. S. and Canadian volunteers for snow operations in northern Norway.
PLUNDER
(March 1945) 21 Army Group assault across the Rhine north of the Ruhr
POINTBLANK
combined bomber offensive against Germany from bases in the United Kingdom
POSTERN
operation to capture Lae-Finschhofen-Madang
PRINCETON
plan establishing the basic outline for the reoccupation of the Visayas-Mindanao-Borneo-Netherlands East Indies area (later renamed MUSKETEER)
PROVIDENCE
plan (canceled) for the occupation of the Buna area of New Guinea
PUGILIST-GALLOP
(1943) attack to outflank the Mareth Line in Tunisia
QUADRANT
U.S.-British Conference at Quebec, August 1943.
QUEEN
12th Army Group operation on the Roer plain between the Wurm and the Roer Rivers
RAINBOW
Various plans prepared between 1939 and 1941 to meet Axis aggression involving more than one enemy. Each plan was named with a color; RAINBOW 5 was published in October 1941
RAINCOAT
assault on the Camino hill mass in Italy
RANKIN
Plans for an emergency return to the Continent in event of a collapse of German resistance.
RASHNESS
revised CORONADO plan
RAVENOUS
plan by 4 Corps for the recapture of northern Burma
RECKLESS
operation to capture Hollandia (1944)
RED
Prewar plan of operations in event of war with Great Britain.
RENO
General Douglas MacArthur's plans for operations in the Bismarck Archipelago, along northern coast of New Guinea and thence to Mindanao, P.I.
RESERVIST
code name for the seizure of vital points at Oran and the landing of Allied forces to prevent sabotage in the harbor
RETRIBUTION
(April-June 1943) plan to prevent Axis forces from reaching Italian territory if they should attempt to evacuate Tunisia
RINGBOLT
operation to capture Tulagi (a sub-task of WATCHTOWER)
ROAST
operation to clear Comacchio Spit, Italy
ROGER
capture of Phuket Island of the Kra Isthmus in Burma
ROMEO
code name for the French commando force that landed at Cap Nègre
ROMULUS
Arakan part of CAPITAL
ROOSTER
operation to fly the Chinese 22d Division to Chihchiang
ROSE
operation (April 1945) in the Ruhr pocket
ROSES
Efate.
ROSIE
code name for the French naval force that landed southwest of Cannes during DRAGOON
ROUNDHAMMER
plan for a cross-Channel operation intermediate in size between SLEDGEHAMMER and ROUNDUP
ROUNDUP
Plan for major U.S.-British attack across the Channel in 1943.
ROYAL MARINE
British proposal for sowing fluvial mines in the Rhine.
RUGBY
code name for the airborne forces dropping behind the beaches during the invasion of Southern France
SATIN
(December 1942) plan (canceled) for II Corps to attack Sfaz, Tunisia
SAUCY
limited offensive to reopen a land supply route from Burma to China
SCIPIO
(6 April 1942) attack by the Eighth Army at Akarit wadi
SEXTANT
code name for the Cairo Conferences (22-26 November and 3-7 December 1943)
SHINGLE
(22 January 1944) amphibious operation at Anzio, italy
SITKA
code name for the task force that captured the islands of Levant and Port Cros in the Mediterranean
SLAPSTICK
(9 September 1943) British airborne landing at Taranto, Italy, mounted from Bizerte, Tunisia
SLEDGEHAMMER
Plan for limited cross-Channel attack in 1942.
SPOONER
New Zealand.
STALEMATE
invasion of the Palaus
STRANGLE
(March-May 1944) air operations to interdict the movement of Axis supplies in Italy
SUMAC
Australia.
SUPER-GYMNAST
Plan for Anglo-American invasion of French North Africa, combining U. S. and British plans and often used interchangeably with GYMNAST.
SUPERCHARGE
(March 1943) revised plan for the assault on the Mereth Line. Also (November 1942) breakout by British 30 Corps in Egypt as part of Eighth Army's offensive in the Western Desert (?)
SWORD
code name for the assault beach at Normandy assigned to British 3d Division
SYMBOL
Casablanca Conference. 14-23 January 1943.
TALISMAN
name for early plan for post-hostilities activities in Germany
TALON
Akyab part of CAPITAL
TARZAN
India-based portion of the general offensive in Burma
TED
code name for a task force operating in the Aitape area of New Guinea
TERMINAL
(8 November 1942) operation in the harbor of Algiers, also code name for the Potsdam Conference (16 July-2 August 1945)
THUNDERBOLT
offensive in the Metz area
TIDALWAVE
code name for the 1943 low level attack on Ploesti, Rumania, by heavy bombers
TIGER
code name for one of the rehearsal exercises for OVERLORD
TOENAILS
operation to capture New Georgia
TORCH
Allied invasion of North and Northwest Africa, November 1942.
TOREADOR
airborne assault on Mandalay
TORNADO
operation to capture Wakde-Sarmi (1944)
TRADEWIND
operation to capture Morotai (1944)
TRANSFIGURE
plan (cancelled) for airborne operations on the west side of the Seine River to block German escape routes
TRIDENT
cover name for the U.S./British Washington Conference (12-25 May 1943)
TULSA
first outline plan by General Headquarters, Southwest Pacific Area, for operations directed at the capture of Rabaul
TWILIGHT
plan to base B-29 bombers in the China-Burma-India Theater of Operations
TYPHOON
operation to capture Sansapor-Mar (1944) in New Guinea
ULTRA
signals intelligence derived from interception and decoding of German ENIGMA machine ciphers
UNDERTONE
(March-April 1945) offensive by Third and Seventh Armies to break through the West Wall and clear the Saar-Palatinate triangle (area within the Rhine, Moselle, and Lauter-Saar Rivers), and to secure a bridgehead east of the Rhine River in the vicinity of Worms
UTAH
code name for the assault beach at Normandy assigned to VII Corps
VARSITY
First Allied Airborne Army operation for crossing of Rhine River in vicinity of Wesel, Germany, in 1945 in support of Operation PLUNDER
VERITABLE
Canadian First Army operation to clear the area between the Maas and Rhine Rivers
VICTOR I
operation in Panay and Negros Occidental
VICTOR II
operation in Cebu, Bohol, and Negros Oriental
VICTOR III
Eighth Army operation against Palawan
VICTOR IV
Eighth Army operation against the Sulu Archipelago and the Zamboanga area of Mindanao
VICTOR V
Eighth Army operations against western Mindanao
VULCAN
(6 May 1943) final Allied offensive to clear Axis forces from Tunisia, particularly the Cape Bon area
WATCHTOWER
operation to capture Guadalcanal and Tulagi (a sub-task of PERSECUTION)
WEBFOOT
rehearsal for SHINGLE
WHITE POPPY
Noumea, New Caledonia.
WILFRED
Plan for laying of mine fields in Norwegian waters.
WOP
(17 March 1943) opening attack by II Corps against Gafsa
X
Australia.
YOKE
code word for all US organizations working with Y-Force in the China-Burma-India Theater of Operations
ZEBRA
code word for US-sponsored Chinese division in east China
ZIPPER
1945 plan for assault on Malaya

Source:
Taken from various publications in the series Report of the Office of the Chief of Military History on the UNITED STATES ARMY IN WORLD WAR II

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