Question: Which country was defeated during World War II that had a submarine in its service called the U-666?

Context articles: United States Submarine Veterans, Inc.
Tambor-class submarine
Gato-class submarine
HNLMS O 15
Davara
Nazi Germany
I-400-class submarine
USS Enterprise (CV-6)
Jack C. Titus
German submarine U-666

The United States Submarine Veterans, Inc. (USSVI) was created by a group of men from the United States Submarine Veterans of World War II.
 They shared a belief in the need of an organization open to all United States Navy submariners, from the very beginning of the Submarine Service to the present and into the future - not limited to just those who served so ably in World War II.
 They wanted to ensure their shipmates who were killed in action on submarines would never be forgotten.
The "Tambor"-class submarine was a United States Navy submarine design, used primarily during World War II.
 It was the USN's first fully successful fleet submarine, and began the war close to the fighting.
 Six of the class were in Hawaiian waters or the Central Pacific on 7 December 1941, with "Tautog" at Pearl Harbor during the attack.
 They went on to see hard service; seven of the twelve boats in the class were sunk before the survivors were withdrawn from front-line service in early 1945; this was the highest percentage lost of any US submarine class.
 "Tautog" was credited with sinking 26 ships, the largest number of ships sunk by a US submarine in World War II.
 The "Tambor"s retained the top speed of 21 kn and range of 11000 nmi (allowing patrols in Japanese home waters) of the preceding "Sargo" class , and improvements included six bow torpedo tubes, a more reliable full diesel-electric propulsion plant, and improved combat efficiency with key personnel and equipment relocated to the conning tower.
 In some references, the "Tambor"s are called the "T Class", and SS-206 through SS-211 are sometimes called the ""Gar" class".
The "Gato"-class were a class of submarines built for the United States Navy and launched in 1941–1943; they were the first mass-production U.S. submarine class of World War II.
 Together with their near-sisters the "Balao" and "Tench" class es, their design formed the majority of the United States Navy's World War II submarine fleet.
 Named after the lead ship of the class, USS "Gato" , the "Gato"s and their successors formed the core of the submarine service that was largely responsible for the destruction of the Japanese merchant marine and a large portion of the Imperial Japanese Navy in World War II.
 "Gato"' s name comes from a species of small catshark.
 Like most other U.S. Navy submarines of the period, boats of the "Gato" class were given the names of marine creatures.
 In some references, the "Gato"s are combined with their successors, especially the "Balao" class.
O 15 was a "O 12"-class submarine of the Royal Netherlands Navy that saw service during World War II.
 It was the only submarine of the "O 12" class built by Wilton-Fijenoord of Rotterdam.
 It was one of many Dutch ships doing convoy duty during the Spanish Civil War.
 When World War II broke out "O 15" was stationed in Curaçao.
 It returned to Europe and was based in Dundee, whence it patrolled the coast of Norway and accompanied convoys to Archangelsk.
 The sub survived World War II and was taken out of active duty just after the Japanese surrender.
 It was demolished in 1946 in Hendrik-Ido-Ambacht.
Davara was a British steam trawler during World War II that was sunk by the German submarine "U-27" .
 On 6 January 1912, the ship was launched from Selby by the shipbuilding company Cochrane & Sons Ltd.
 The ship's owner, Mount Steam Fishing Co Ltd. christened her "Davara".
 By March 1912, the trawler was registered and completed and in 1914 she was requisitioned by the British Royal Navy for service in World War I and fitted out for mine sweeping activities.
 Having survived World War I, she was returned to her owners in 1919 and served as a trawler once more until 13 September 1939 when she was sunk by 35 rounds from "U-27" 's main deck gun.
 The 12 man crew managed to escape the submarine in a lifeboat while the "Davara" later sunk at 14:55 from the damage inflicted by "U-27"' s gunfire.
 Her crew remained in the water "baling and rowing" for five hours before they were picked up by the steamer "Willowpool" and safely made landfall.
 "Davara" was the first British trawler to be sunk by enemy action in World War II.
Nazi Germany is the common English name for the period in German history from 1933 to 1945, when Germany was governed by a dictatorship under the control of Adolf Hitler and the Nazi Party (NSDAP).
 Under Hitler's rule, Germany was transformed into a totalitarian state in which the Nazi Party controlled nearly all aspects of life.
 The official name of the state was "Deutsches Reich" from 1933 to 1943 and "Großdeutsches Reich" ("Greater German Reich") from 1943 to 1945.
 The period is also known under the names the Third Reich (German: "Drittes Reich" ) and the National Socialist Period (German: "Zeit des Nationalsozialismus" , abbreviated as "NS-Zeit").
 The Nazi regime came to an end after the Allied Powers defeated Germany in May 1945, ending World War II in Europe.
The I-400"-class submarine (伊四百型潜水艦 , I-yon-hyaku-gata sensuikan ) Imperial Japanese Navy (IJN) submarines were the largest submarines of World War II and remained the largest ever built until the construction of nuclear ballistic missile submarines in the 1960s.
 The IJN called this type of submarine Sentoku" type submarine (潜特型潜水艦 , Sen-Toku-gata sensuikan, "Submarine Special" ) .
 The type name was shortened to Toku-gata Sensuikan (特型潜水艦 , Special Type Submarine ) .
 They were submarine aircraft carriers able to carry three Aichi M6A "Seiran" aircraft underwater to their destinations.
 They were designed to surface, launch their planes, then quickly dive again before they were discovered.
 They also carried torpedoes for close-range combat.
 They are considered the strategic predecessors to today’s ballistic submarines, especially to the Regulus missile program begun about a decade after World War II.
USS "Enterprise" (CV-6), was the seventh U.S. Navy vessel to bear the name.
 Colloquially called "the Big E", she was the sixth aircraft carrier of the United States Navy.
 A "Yorktown"-class carrier, she was launched in 1936 and was one of only three American carriers commissioned before World War II to survive the war (the others being "Saratoga" and "Ranger" ).
 She participated in more major actions of the war against Japan than any other United States ship.
 These actions included the Attack on Pearl Harbor (18 dive bombers of VS-6 were over the harbor, 6 were shot down with a loss of eleven men, making her the only American Aircraft carrier with men at Pearl Harbor during the Attack and the first to receive casualties during the Pacific War), the Battle of Midway, the Battle of the Eastern Solomons, the Battle of the Santa Cruz Islands, various other air-sea engagements during the Guadalcanal Campaign, the Battle of the Philippine Sea, and the Battle of Leyte Gulf.
 "Enterprise" earned 20 battle stars, the most for any U.S. warship in World War II, and was the most decorated U.S. ship of World War II, She is also the first American ship to sink an enemy vessel during the Pacific War, the sole surviving pilot of the six planes shot down over Pearl Harbor sank Japanese submarine I-70 on 10 December 1941.
 On three occasions during the Pacific War, the Japanese announced that she had been sunk in battle, resulting in her being named "The Grey Ghost".
Jack Clarence Titus (September 13, 1909 – December 21, 1985) was a submarine commander during World War II and the Korean War who reached the rank of Captain in the United States Navy.
 Captain Titus appeared as a guest on an episode of the syndicated television series Silent Service called "Narwhal's Passenger From Mindanao," which detailed a rescue operation involving USS Narwhal under his command.
German submarine "U-666" was a Type VIIC U-boat built for Nazi Germany's "Kriegsmarine" for service during World War II.