According to Wikipedia, Japan attacked Pearl Harbour to prevent the US Pacific Fleet from interfering with its actions in South-East Asia.
However, Noam Chomsky, referring to Michael Sherry's The Rise of American Air-Power, relates that:
The Japanese knew the B-17 flying fortresses were coming off the production lines of Boeing and could read in the American press that planes capable of burning down Tokyo, "a city of rice-paper and wood houses". A November 1940 plan to "bomb Tokyo and other big cities" was enthusiatically recieved by the Secretary of State, Cordell Hull. FDR was "simply delighted" at the idea - described graphically by its mastermind, air force general Claire Lee Chennault: to "burn out the industrial heart of the Empire with fire-bomb attacks on the teeming bamboo anthills of Honshu & Kyushu."
By July 1941, the air-corps was ferrying B-17s to the Far East for this purpose, moving half of all the big bombers from the Atlantic sea-lanes to this region. If needed, the planes would be used "to set the paper cities of Japan on fire," General George C. Marshall explained in a confidential press briefing on 15th November 1941, adding "there won't be any hesitation in bombing civilians." Four days later, New York Times senior correspondant, Arthur Krock, presumanly basing himself on Marshall's briefing, reported US plans to bomb Japan from Siberian and Phillipine bases, to which the air force was rushing incendiary bombs intended for civilian targets. Washington knew from decoded messages that Japan was aware of the dispatch of B-17s.
The attack by Japan on Pearl Harbour was on the 7th December 1941.
Hence the question:
Q. Why was the the USA planning to attack Japan before the Japanese attack on Pearl Harbour?