The vote to continue conscription passing by only 1 vote in 1941 was inevitably interpreted by some that the US was not really willing to fight, along with anything else that could be fitted to this conclusion. With a still peacetime press there were plenty of stories of US unreadiness, unwillingness and failures.
The Tripartite Pact (Written in English, the US soon had a copy) from 27 September 1940 committed Japan to attack the US if the US attacked Germany and/or Italy and vice versa. Lots of people were sure the US would enter the European war "soon", thinks like the transfer of destroyers, Lend Lease etc. The way Britain let the US lead negotiations with Japan, trying to convince Japan they were going to fight together. Better to start the fighting yourself that be on the receiving end.
Taranto Raid 11 November 1940, a Japanese naval officer was allowed to do an inspection.
A Japanese nightmare was a collapse into civil war resulting in Japan being colonised. The level of violence in Japanese politics made the possibility seem very real. As of 1940 the existing Japanese leadership was tied to the China War, they would have to go before a change in policy but given the radicals, both civil and military, a change in war policy would definitely result in violence and assassinations. The Japanese rule from below concept, "I am their leader, I must follow them."
Whether the US could or should enter the European War was a live political issue in 1941, if the US announces Japan is thinking of attacking Pearl Harbor there will be strong demands to prove it, where is the evidence, wonder what will happen if the answer is Peruvian Ambassador or that the idea has been dismissed by US military officials.
1) the IJN could not put a large carrier fleet into Hawaiian waters except from the mandated islands, according to intelligence.
2) Pearl had enough search aircraft to guard this approach.
3) Pearl Harbor had 161 out of the USAAF total of 1,618 fighters at the end of November 1941 (the Philippines had 228, Latin America 231 according to the USAAF Statistical digest). Pearl had several radar sets available, plus the radars on USN ships.
4) Until Shokaku and Zuikaku could be considered operational the carrier raid from the north was 1 IJN carrier.
5) Until at sea refuelling was possible the carriers coming from the north would have minimal escorts.
6) The possibility of an airstrike was always on the table, the point was how probable, how much to tie Pearl Harbor up on that possibility, cutting into the time and material needed for other things like training.
7) The "best" use for the IJN carriers was covering the landings in South East Asia (like the IJN Naval General Staff wanted) or waiting to counter USN moves.
8) The known IJN war plan was to attrition the USN as it advanced, the carriers and most battleships in home waters waiting to engage the advancing USN fitted into this.
9) Pearl Harbor was attacked by submarines.
The IJN fleet had to learn refuelling at sea to make it, using civilian tankers which tended to require rounding up every morning, and the Naval General Staff spent a lot of time refusing to allocate enough tankers to enable 6 carriers to go.
It should be remembered that during 1940 and 1941 there were plenty of IJN officers prepared to, in effect, state in Japan versus the world bet on Japan sort of attitudes. There were all sorts of rumours about what might happen and plenty of warnings that had turned out to be false.
If I understand Edwin Layton correctly the warning was the Peruvian minister had been told "from many sources, including a Japanese source, that in the event of trouble's breaking out between the United States and Japan, the Japanese intended to make a surprise attack against Pearl Harbor with all their strength and employing all their equipment."
Admiral Yamamoto had handed his idea to attack Pearl Harbor to the Navy Minister on 7 January 1941, as part of a 9 page report called "Views on preparations for War". It was a series of suggestions. The idea it was a leak is implied because Grew reported the information on 27 January, three weeks after Yamamoto reported his ideas. Yamamoto advocated a total change in strategy, from allowing the US to advance, to seeking out the US including an air attack on Hawaii and a submarine blockade. A copy of Yamamoto's idea had gone to Admiral Onishi for study and evaluation. Onishi came back 6 weeks later, so 3 weeks after Grew's message, to report the operations was feasible if surprise was achieved. Commander Genda then spent 2 months to work out the tactical plan. In other words Yamamoto's paper was a suggestion paper not a detailed plan, about what the IJN should evaluate as initial war plans. After apparently being widely leaked and also exaggerated when the plan was only known by 3 senior IJN Admirals there were no further leaks for the next 10 to 11 months.
Now to evaluate Grew's message, all strength and all equipment, if true must mean an invasion. The rumour does not say a major raid, it says all out attack. It also says Grew is reporting second hand and that the majority of the sources are other diplomats. Ever done the pass the message test, where a chain of people whisper a message to each other and you observe the difference between the first and final versions of the message? Already, if we are to believe it was a real leak of Yamamoto calling for an air strike, it has escalated to most of the combined fleet supporting a major invasion. A USN evaluation talks of disposition of Japanese Naval and *Army* forces, how "the present" disposition rules it out, absolutely correct.
Layton notes that a Japanese author had published the idea of an air attack on Pearl Harbor in the mid 1930s, apparently in response to the USN fleet problem that demonstrated the concept.
The famous things are going to happen automatically message gave the cut off date as 29 November 1941, the 30th was a Sunday, so some of the US Army intelligence people put out a prediction of an attack on the 30th, when it did not come it meant future predictions were treated with greater reserve.
As another example on the 23rd of September 1941 the Japanese consulate in Buenos Aires sent a message to Tokyo stating it had received copies of Italian maps of the Panama Canal Zone and was sending them by courier to Japan. This message was translated by the US on 22 October. Admittedly the message seems to suggest the gaining of the maps was fortuitous, not as part of a (much) earlier request. Should Panama have been alerted, and if so in what way and what should they do? One hypothetical US Japan war scenario begins with Japanese merchant ships blowing themselves up while in the Panama Canal locks.