 
<translate> Yi amfani da rafi na sani don amsa wadannan. Shirye-shiryen farko don kai hari kan Pearl Harbor don kare motsi zuwa "Yankin Kayan Kayan Kudancin" (kalmar Jafananci ga Dutch East Indies da kudu maso gabashin Asiya gaba ɗaya) ya fara ne tun farkon 1941 a ƙarƙashin jagorancin Admiral Isoroku Yamamoto, sannan ya ba da umarnin Sojojin Jirgin Ruwa na Japan. Ya sami amincewa da tsari na yau da kullun da horo don kai hari daga Babban Jami'in Sojan Ruwa na Imperial Japan kawai bayan jayayya da yawa tare da Babban Ofishin Sojan Ruwa, gami da barazanar yin murabus daga mukaminsa. An fara cikakken shiri a farkon bazarar 1941, da farko daga Rear Admiral Ryūnosuke Kusaka, tare da taimakon Kyaftin Minoru Genda da Mataimakin Shugaban Ma'aikatan Yamamoto, Kyaftin Kameto Kuroshima. Masu tsarawa sun yi nazari sosai game da harin jirgin sama na Burtaniya a 1940 a kan jirgin ruwa na Italiya a Taranto.  Amsa wannan tambaya bisa ga nassi: Yaushe ne Japan ta fara shirin kai hari a Pearl Harbor? <eng> Use stream of consciousness to answer the following. Preliminary planning for an attack on Pearl Harbor to protect the move into the "Southern Resource Area" (the Japanese term for the Dutch East Indies and Southeast Asia generally) had begun very early in 1941 under the auspices of Admiral Isoroku Yamamoto, then commanding Japan's Combined Fleet. He won assent to formal planning and training for an attack from the Imperial Japanese Navy General Staff only after much contention with Naval Headquarters, including a threat to resign his command. Full-scale planning was underway by early spring 1941, primarily by Rear Admiral Ryūnosuke Kusaka, with assistance from Captain Minoru Genda and Yamamoto's Deputy Chief of Staff, Captain Kameto Kuroshima. The planners studied the 1940 British air attack on the Italian fleet at Taranto intensively.

Answer this question based on the passage: when did japan start planning the attack on pearl harbor? 