Salazar denounced the allied operation as an invasion of a neutral territory. On 20 February 1942, alleging self-defence, Japan invaded the island of Timor. From December 1942, German merchant ships which had sought refuge in the territory of Goa in Portuguese India in 1939 began providing regular intelligence on Allied shipping via radio to German U-boats of the Monsun Gruppe operating in the Indian Ocean. Once the British discovered this, the Special Operations Executive launched Operation Creek, using a group of part-time soldiers called the Calcutta Light Horse to infiltrate the harbour and sink the German ship "Ehrenfels" with limpet mines, ending the transmissions. Due to the violation of Portuguese neutrality, the operation was kept secret until 1978. The Germans had planned an attack on Gibraltar, codenamed Operation Felix, which was never initiated. It included the potential invasion of Portugal if the British gained a foothold and considered the occupation of Madeira and of the Azores. On 12 November 1940 Hitler issued Führer Directive No. 18, which outlined the plan to invade Portugal if British forces were to gain a footing there. "I also request that the problem of occupying Madeira and the Azores should be considered, together with the advantages and disadvantages which this would entail for our sea and air warfare. The results of these investigations are to be submitted to me as soon as possible," Hitler added. In June 1941, Operation Isabella was a Nazi German plan to be put into effect after the collapse of the Soviet Union to secure bases in Spain and Portugal for the continuation of the strangulation of Great Britain. This concept was laid out by Hitler, but was never executed. Operation Alacrity was the codename for a proposed Allied seizure of the Azores during World War II. The islands were of enormous strategic value with regard to the defeat of the German U-boats. Salazar was especially worried about a possible German invasion through Spain and did not want to provoke Hitler; nor did he want to give Spain an excuse to take sides with the Axis and invade Portugal due to the strategic importance of the Canary Islands. Great Britain and the United States devised plans to set up air bases regardless of Portugal's disapproval. The plans were never put into operation. Instead in 1943 Britain requested, and Portugal agreed, to allow Britain to set up bases there. Operation Alacrity was preceded by War Plan Gray. War Plan Gray was a plan for the United States to invade the Azores Islands in 1940–41. Gray is one of the many color-coded war plans created in the early 20th century.