This was a British unrealised operation by the Special Operations Executive to intercept and capture the Japanese auxiliary armed merchant cruiser and intelligence-gathering vessel Asaka Maru (8 February 1941).
The vessel carried a number of Imperial Japanese navy intelligence personnel tasked with gaining information about the US and British defences at Pearl Harbor, in the Panama Canal area, and at Gibraltar. The 7,399-ton refrigerated cargo vessel had been completed in November 1937, and on 24 December 24 December 1940 had been requisitioned by the Imperial Japanese navy for an initial part-conversion at Yokosuka into an auxiliary armed merchant cruiser. This conversion was completed on 15 January 1941 and included two 120-mm (4.72-in) guns. The vessel departed departed Yokohama on 16 January carrying a 40-man ‘Naval Inspection Group’ comprising 24 naval officers and 16 civilians assigned to Germany and including as leader Vice Admiral Naokuni Nomura as well as Rear Admirals Katsuo Abe, Naosaburo Irifune and Sakamaki Munetaka, and Captains Namizo Sato, Kozo Nishina and Minoru Matsuo.
On 7 February the vessel reached the Panama Canal. The US authorities initially demanded to search the ship during her transit through the canal, relenting only after lengthy negotiations. On 8 February the British foreign secretary, Anthony Eden, sent a memorandum to the cabinet drawing attention to the ship. In the following days, Lord Halifax, the British ambassador to the USA, contacted President Franklin D. Roosevelt, who advised against interception of the Japanese vessel, an opinion with which the British Admiralty concurred. Prime Minister Winston Churchill then asked the Special Operations Executive-Far East to consider an ‘M’ (i) plan to seize or destroy the ship. In a series of cabinet meetings on 10, 13, 20 and 24 February, and 6 and 13 March, a blockade of Asaka Maru was discussed.
On 9 February the Japanese vessel departed Cristóbal, Panama, toward Europe via the Mona Passage, reaching Lisbon, Portugal, on 20 February. Here she disembarked the Naval Inspection Group, which travelled overland to arrive in Berlin four days later. On 24 February the ship departed Lisbon and four days later arrived in the northern Spanish port of Bilbao, where she loaded 3,000 tons of munitions and supplies from Germany and Switzerland, including 20-mm Oerlikon cannon for fighter aircraft, cases of machinery, machine tools, mercury, electronic devices, etc.
On 2 March Roosevelt ordered that every foreign ship passing through the Panama Canal be inspected. On 13 March the Japanese vessel departed Bilbao for Japan via the Cape of Good Hope to avoid the Panama Canal. On 20 March the British invited the Dutch government-in-exile to conform to British contraband control procedures, which included the affording of no facilities unless the Japanese ship submitted to these procedures: if Asaka Maru refused to comply, she was not to be allowed to take supplies on board, or to remain in port. On 12 April the ship entered the Flores Sea through the Lombok Strait, then made for the Molucca Passage. On 22 April the ship entered Tokyo Bay and anchored at Yokohama.