A) Recall event of 22June1941.
B) Japan needed oil, and after a world-wide embargo became effective (thank you FDR and Churchill at Placentia Bay).
C) Japan always "gamed" their moves. The gaming of their move south (say NEI and oil), Pearl Harbor as a threat to their flank is seen. Hart and his "Asiatic Fleet" was not seen as a major force - largest warship is CA.
D) Japan knows, via the captured Blue Funnel streamer, from the COS papers that Britain is in no position to sustain a naval offensive in the Far East. (See Chapman's The Price of Admirality). The only worry - the Pacific Fleet, now lessened because FDR move major elements to Altantic - that convoying thing. Puzzle - how many fleet oilers remain at Pearl Harbor?
E) The US via FDR, committed to the armed support of British and Dutch terrority - unconstitutionally and not revealed until the Pearl Harbor Hearings
F) There was no treaty to defend Britain or the Dutch - nothing ratified by the US Senate.
G) Consider international law as well as the US being a declared neutral country at the time. And what that meant.
... And so on.
Timing is everything.
I wonder if he was underestimating the Japanese as many (foolish) Japanese underestimated us?