Actually no. The Pacific fleet and US Army units in the Pacific had been issued a order to consider any Japanese encountered as hostile and to attack on sight. I cant recall the exact date this order was issued, but it came shortly after negotiations collapsed in November. This order was similar to the 'attack on sight' order given to the USN for enforcing the Atlantic nuetrality zone against Axis warships. Hence the attacks on German submarines previous to the German DoW on the US in 1941.
Yeah, the idea being that having a surprise discovery of what compromises Japan's entire fleet carrier force that close to Oahu can be considered a legitimate
Casus Belli. Its the equivalent of the USA discovering a major Soviet amphibious invasion force off the coast of Norfolk during the Cold War. ASB, I know. But then wouldn't the raid on Pearl Harbor have been considered ASB by the contemporaries of 1941?
And if the Japanese are floating really close to American waters, wouldn't that raise a flag of alarm in the American mind?
Yes, to the point of a wolf pack being caught at the mouth of the lion's den.
So in this scenario, the US will be the aggressor? Would it be only a show-of-force "border skirmish" or will US declare war on Japan?
The Japanese have ZERO excuse for having their main carrier fleet getting so close to America's main fleet base in the Pacific. What you describe would apply if the
Kido Butai was found by a US submarine 100 nmiles off of Truk and the USN "decided to teach them a lesson".
Two possibilities here that could occur.
- Nagumo decides surprise has been lost and turns the attack force around. There is no day of infamy. [1] On the plus side the U.S. does not enter the war at this point, whether it would later is open for debate. [2] The Japanese move into Malaysia, Burma, Hong Kong, etc. goes ahead minus the attacks on the Philippines which are aborted in order to prevent the entry of the U.S. into the war. [3] This delays U.S. entry into World War II by several months. Given anti-war sentiment at home FDR would probably not declare war to save British interests (the anti-war faction claimed that Britain was willing to "fight to the last drop of American blood"). [4] Eventually it is likely German submarine attacks force FDR to finally request a declaration of war but the war doesn't receive anything like the support it does in OTL since we have no outrage from a surprise attack on Pearl Harbor and no approx. 3,000 servicemen and civilians dead. Lack of U.S. support means the British have to endure several months more of a crippling submarine blockade and Russia likewise has no Second Front until later in the war. This will probably mean a prolonged World War II [5].
1] The war is already on.
2] Aborting the Pearl Raid doesn't stop the raid on the PI. And US entry into WWII means Total Mobilization, far better for Britain that the loss of what the US would expend in the Pacific.
3] That'll get the assassinations going in Tokyo. By the end of 1942 the PI will be invulerable to Japanese assault, by 1943 FDR can cut off the DEI/Malaya oil convoys at will, by 1944 the Two Ocean Navy will be complete and Japan will be helpless beyond a first few rounds of seeing its fleets destroyed.
4] Isolationist sentiments were waning over time, not growing. Even following your scenario, at best OLL will be flowing to both the USSR and the UK. And Japan can't go to war with the British without sparking a war with the US.
5] WWII is not Vietnam, much less the various wars in the Middle East.
1. Nagumo decides to attack the U.S. force rather then totally waste the moment. Since the U.S. force has no carrier support the attack by Nagumo is successful. Most of the U.S. force is sunk, this time with no chance of recovery of any of the ships lost, and possibly a much larger loss of life. As indicated earlier the planes at Pearl Harbor are not destroyed which means the U.S. still has an air strike force. It is possible they could be readied and launched if word got back to PH that an attack was under way. How successful they would be remains to be seen. In any case FDR gets a "day of infamy", just in a different way from OTL. How things go from here I leave to further discussion.
Geon
Since the
Kido Butai was flying NO aerial reconnaisance (for fear the aircraft themselves would be sighted and blow strategic surprise) Nagumo has no better prospect of tactical surprise in this scenario than do the Americans, especially in a night engagement. IIRC, the Americans have some radar, while the Japanese have none.
Task Force 1 (the Battle Force) is not going to get wiped off the face of the sea without sending warnings to Pearl.
If the Americans are in sight of the Japanese Carriers
And the Americans have orders to treat them as hostile
The Carriers are toast......
If its Japanese spotter planes that locate the American Battleships they are toast....
No spotters in flight, as OTL
IMO, the most likely outcome.
He MUST continue at this point, and the Japanese had already calculated for the loss of a third of their force...
Unlikely, since the P.I. were a clear threat to SLOCs for the Southern Op, hence the need to attack the U.S. in the first place.
Exactly. The whole basis of the Southern Strategy was built upon safe LOCs
<snip> IMO, the best outcome for Britain is the U.S. not entering the war at all. That way, all the supplies otherwise destined for U.S. forces are available for Brit/Commonwealth...
The UK needed the US in the war all out more, unless you're looking for a Warsaw Pact that extends to Portugal!
Given U.S. involvement cost Coastal Command dozens of precious VLR Liberators OTL, this scenario, by making them available, is better for Britain, not worse.
Having 15000

Liberators built overall for the war effort is even better!
And many obsolete & obsolescent aircraft remain in U.S. service that were otherwise replaced by P-38s, so not an unvarnished good.
Not likely, as you will have a number of pilots who will survive OTL, plus except for the P-40s these were VERY obsolescent fighters, which was not totally unknown OTL. Though I'll admit that the employment of the Brewster Buffalo at Midway suggests otherwise.
In case it's pertinent to the outcome, let's say that the admiral in charge of the U.S. Force encountering the IJN carrier fleet is vice-admiral William S. Pye, who as far as I can determine was the commander of the USN Pacific Fleet 'Battle Force' at the time. (Will update the opening post with this.)
Pye's brief command of CincPac prior to Nimitz's arrival does not inspire confidence. Then again, that was OTL post-Pearl.
NOT "good flying weather"... certainly not the farther north one is, even today, and more so in the 1940s.
Depending upon when this encounter takes place during the Japanese approach to Oahu (on Dec. 6 they were probably within ~400 miles nne of Kauai), and (huge IF) the US force is truly ready to shoot on sight (as the Atlantic Fleet was at the time of the Kearny and Reuben James incidents), the Japanese could suffer heavily ... carriers rarely did well in gunnery actions, after all. Just ask Glorious and Gambier Bay...
The only defense Nagumo had was speed, but does he turn tail and put his carriers into the wind before his lookouts can determine what may be nothing more than a Coast Guard cutter or fishing boat?

Otherwise, his own escorts could easily deal with such a threat, and Pearl would still only have at best a minimal (and brief) radio warning.
IIRC, Nagumo's orders were to abort if the IJN task force was discovered east of the deadline up to a certain date; I do not recall if it was the date of the attack, or prior, however.
Best,
By the time of this scenario (within 400 nm?) he's committed.
Battleships vs Carriers in bad flying weather?
Bye-bye Kido Butai...
Especially if its before they can refuel. If the tankers are lost before then, Nagumoo's whole fleet is lost.
Yep...The Battle Force would have had 16 16 inch rifles, and 44 14 inch, in the main batteries of the six battleships, as discussed above; the IJN CBs would have had 16 14 inch guns.
Nagumo had all of 9 destroyers (3 covering the tankers), 1 light cruiser (Abukuma), 2 heavy cruisers (Tone & Chikuma), and 2 battlecruisers (Kirishima & Hiei) covering his carriers. ITTL he's up against (IIRC) at least four heavy cruisers, one light cruiser, 20+

destroyers, plus the Battle Force (7 battleships-
California, Oklahoma [6], Arizona, Nevada, Maryland, Tennessee, West Virginia).
IOW,
6] Pye's flagship
As said, Colorado is on the West Coast and Pennsylvania is in drydock
60 to 14, or 4 to 1 odds...and depending on time of day/night, relative locations, and a host of other factors, the largest obvious targets could very well be one or more of the IJN's carriers...which are fast, but not that fast, and loaded with several hundred very combustible aircraft.
Depending on your POV, it all depends on whether you:
go to the extremes of a night action in which the USN has spotted the enemy by radar, crossed the enemies' T, sent in destroyer attacks to disorganize the enemies' screen, and then pull a Surigao Strait,
OR
Have Nagumo spot the enemy in daylight, instantly determine who they are, turn into a favorable wind while the screening force sacrifices itself launching a deadly torpedo attack against Pye, who is confused and falls into indecision, then the Battle Force is disorganized as the
Kido Butai's strike force has the time to escape, arm for flight, launch, and nail the USN's battleline, resulting in total loss on the high seas. Only the 7th US Air Force is spared, as a strike on Oahu itself is now superfluous.
Basically, it probably becomes a melee, with the US battleships forming a line, the cruisers and destroyers from both sides mixing it up in the middle, the Japanese battlecruisers "trying" to hold off the Americans battleships, and the IJN carriers - less any that get hit in the initial encounter, hauling out for points north at top speed.
Agreed. Though the Japanese carrier's high speed is very good, its not like they can instantly "go to warp" either. Events showed in the Mediterranean what could happen when lightly protected but very fast warships ran into tougher but slower enemy vessels. Methinks ITTL the Kaga and Akagi will have slightly a harder time of escaping than the newer ships.
If the wind is from the "wrong" quarter, the ability of the IJN to launch aircraft and/or withdraw from out of gunnery range is going to be hampered, to some degree, or even absent; likewise, if the USN and IJN start using smoke, who knows if anyone can hit much of anything? The Komandorskis would be the closest equivalent, and there were no carriers involved...
At the Komandorskis the Japanese outnumbered and outgunned the USN forces two to one, yet thanks to the Japanese commander's caution he never closed in and destroyed the American force. Which is why Hosogaya got canned out of the IJN by Yamamoto when he got back, while the American admiral and his crews were treated to a ticker-tape parade when they returned to Washington State!
So I guess it depends on who is the more agressive...
And that was in the daytime. If it is a night action, the IJN carriers have nothing to contribute beyond getting the hell out of Dodge.
In which case Pearl Harbor becomes a national humiliation of retreat for Japan, with their primary strike force doing a brief Hawaiian Excursion while the rest of the navy (and Army

) do all the fighting. Who gets a trip to the Imperial Garden? Nagumo or Yamamoto?
Given the set-up, there are so many potential deltas the end result could be a clean sweep for either combatant.
Best,
Yep