Japanese submarine Ro-66

Upon commissioning, Ro-66 was attached to the Sasebo Naval District and assigned to Submarine Division 27 — in which she spent her entire career — in Submarine Squadron 1 in the 1st Fleet in the Combined Fleet.[2][3] Submarine Division 27 was reassigned to the Sasebo Defense Division in the Sasebo Naval District on 30 November 1929.[2] On 24 December 1929, the division was reassigned to Submarine Squadron 2 in the 2nd Fleet in the Combined Fleet.[2] Ro-66 was decommissioned and placed in reserve at Sasebo, Japan, on 15 November 1930.[2][3]

Ro-66 was recommissioned on 1 May 1931,[2] resuming active service in Submarine Division 27, which by then had been reassigned to the Sasebo Defense Division in the Sasebo Naval District.[2] On 1 October 1932, the division was reassigned to the Sasebo Guard Squadron.[2] On 15 November 1933, the division returned to service in the Sasebo Defense Division,[2] and that day Ro-66 again was decommissioned and placed in reserve at Sasebo.[2][3]

Ro-66 was recommissioned on 15 November 1934,[2] and returned to service in Submarine Division 27 which by then again was serving in the Sasebo Guard Squadron in the Sasebo Naval District.[2] The division was transferred to the Sasebo Defense Squadron on 15 November 1935.[2]

Sources offer different accounts of Ro-66′s status during 1938–1940, implying both that she was in full commission throughout those years[3] and stating that she was out of commission and in reserve — in Second Reserve from 15 December 1938 to 15 November 1939 and then in First Reserve until 15 November 1940[2] — in the Sasebo Naval District between December 1938 and November 1940.[2] Whatever the case, she was back in active service in time for Submarine Division 27′s reassignment to Submarine Squadron 7 in the 4th Fleet in the Combined Fleet on 15 November 1940.[2]

When the Imperial Japanese Navy deployed for the upcoming conflict in the Pacific, Ro-66 was at Kwajalein in the Marshall Islands with the other submarines of Submarine Division 27, Ro-65 and Ro-67.[3] She received the message "Climb Mount Niitaka 1208" (Japanese: Niitakayama nobore 1208) from the Combined Fleet on 2 December 1941, indicating that war with the Allies would commence on 8 December 1941 Japan time,[3] which was on 7 December 1941 on the other side of the International Date Line in Hawaii, where Japanese plans called for the war to open with their attack on Pearl Harbor.

On 6 December 1941, Ro-66 got underway from Kwajalein with the commander of Submarine Squadron 27 embarked to conduct a reconnaissance of Wake Island,[3] which the Japanese planned to invade on 8 December 1941, the first day of the war on Wake Island′s side of the International Date Line. The Battle of Wake Island began on 8 December 1941, with Ro-65, Ro-66, and Ro-67 patrolling in the area supporting Japanese forces attempting to seize the atoll.[3] United States Marine Corps forces defending Wake drove back the initial Japanese assault that day.[3]

While the Japanese gathered reinforcements for a second and larger invasion of the island, Submarine Squadron 7 sent orders to all three submarines of Submarine Division 27 on 12 December 1941 directing them to return to Kwajalein.[3] Accordingly, Ro-65 and Ro-67 headed back to Kwajalein, but a radio failure prevented Ro-66 from receiving the orders despite three attempts by Submarine Squadron 7 to contact her.[3] Consequently, she continued to patrol off Wake Island[3] as the submarines of Submarine Division 26 — Ro-60, Ro-61,and Ro-62 — arrived in the area to relieve the departing submarines.[4][5][6]

Ro-66 was on the surface 25 nautical miles (46 km; 29 mi) southwest of Wake Island — bearing 252 degrees from the atoll — to recharge her batteries in a heavy squall in the predawn darkness of 17 December 1941 when her lookouts suddenly sighted Ro-62, also on the surface and recharging batteries.[2][3][6] Both submarines attempted to back off, but it was too late to avoid a collision, and Ro-62 rammed Ro-66 at 20:20 Japan Standard Time.[3][6] Ro-66 sank at 19°10′N 166°28′E[2] with the loss of 63 lives, including that of the commander of Submarine Division 27.[2][3][6] Ro-62 rescued her three survivors, who had been thrown overboard from her bridge by the collision.[2][3][6]

The Battle of Wake Island concluded on 23 December 1941 with the Japanese conquest of the atoll.[3] The Japanese struck Ro-66 from the Navy list on 15 January 1942.[3]