I am not sure what the tensions / political situation was like in late 1940 vs 1941. I am also not sure what the American assets stationed at Peal Harbor were in 1940. There may be items which preclude this.

If the attack is possible, how about a Japanese attack on Pearl Harbor one year earlier than historically occurred. The Japanese would be missing two carriers but might have been able to substitute two light carriers. The British carried off such a raid against the Italians with biplanes. What could the Japanese have arrayed against the Americans?

Also, What kind of effect might it have had on how long it took the United States to ramp up for war, how would it have effected any American involvement in the war, and how might it have affected warship development?

My thoughts are that at least as far as Warship Development, the Midway class carriers might have not been developed. The Essex class carriers appear to have already been close to ready to begin construction. USS Essex herself might have been build as a Yorktown class though and laid down a month or two earlier than she was. Radar was already under development so that would have been still important in the war.

I'm not sure that it woud have made a damn bit of differance because we were effectively at war anyway with U-boats sitting off the USA coast by 1940.

There seem to have been few, if any merchant ships sunk west of the mid Atlantic until after December 1941. The USN wasn't actively involved in ASW operations against the Germans until 1941.

Moreover, there is much more to mounting a sub campaign than 'sitting off the USA coast' -- which is why nothing happened until Fall Paukenschlag. The German Navy was taken by surprise by the German declaration of war on the US, just as much as the opening of the war in 1939. 30 of 90 available boats were in the Mediterranean. Doenitz wanted to concentrate boats off the US coast and take advantage of predictably weak US ASW defenses and readiness for war. But he had to settle initially for only 6 boats, arriving mid-Jan42. They were successful for six months [400+ ships, 2M tonnes-- almost 10% of the war's toll], but by May42, the boats were back in mid-Atlantic positions, and that was the main campaign from June onward, and the site of the greatest convoy battles of the war. The first U-tanker was not off the US until late April, doubling the effectiveness of the attack boats, but the latter never exceeded 15 at one time. The small size, capacity of the "Atlantik U-boot" limited the regions where submarine warfare could be waged in the required force.

The Japanese would not have dared launch the Pearl Harbor attack they historically did, without Mitsubishi A6M2 Zero fighters in the attack force; I don't think they would have had enough of these in December 1940.

Zeroes (Model 11's) were used in trial quantities in combat in China from September 1940, so small complements could have been available to augment the Type 96's on the carriers, though they didn't have the folding wingtips to be stowed in hangars.

But the USN carrier force was also still mainly equipped with 1930's fighters in Dec 1940. As of June, 10 F2A-1's partly equipping VF-3 on Saratoga were the only operational monoplane fighters in the USN, less numerous than Dec 1940 Zeroes. The rest were F2F and F3F. I don't believe that changed until early 1941, when F2A-2's arrived for VF-3 and VF-2 on Lexington. F4F-3's entered service also around the turn of the year but in the Atlantic first, VF-41 on Ranger.

The Type 96 wasn't obviously less of a match for the Grumman retractable undercarriage biplanes than the Zero was for the F4F. The Type 96's lesser range (or more accurately lack of the Zero's phenomenal range) though would have required changes in the JNAF's early war land based fighter operations in places like Philippines, DEI and Malaya; it would have influenced carrier operation also though to a lesser extent.

Another perhaps more important factor in Japanese readiness to attack PH were Type 91 Mod 2 torpedoes modified for shallow water attack, something actually only developed in 1941. Also, the Type 99 No. 80 AP bomb, as used in horizontal bombing v BB's at PH, was adopted in 1941. A late 1940 attack has to theorize acceleration of the efforts to come up with that specialized ordnance.

Joe