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This question already has an answer here:

Conspiracy theorists discuss Pearl Harbor all the time, implying it was planned as an excuse for the war or that we knew it was coming. Well, let's say that we did discover it in time and were able to stop Pearl Harbor from happening by moving our forces out to be ready to defend it. Japan saw the redeploy, figured that Pearl Harbor wasn't the easy victory they were looking for, and decided to have their fleet sail on past, pretending the fleet was always headed somewhere else and was never intended to attack Pearl Harbor. No major incident happened, no shots fired, and no proof that any attack was every going to happen.

How would this affect WW2? In particular how would it draw out the war or affect how it would end?

One obvious relevant question to this would be whether or not the US would join the war at all or sit the entire thing out. You're welcome to hypothesize either that the US never joined or that it does so at some later date. I'm mostly interested in the final outcome of the war regardless of US involvement, but obviously addressing the later US war efforts may influence the final answer somewhat.

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marked as duplicate by Mark, Jim2B, Dan Smolinske, Frostfyre, clem steredenn Dec 4 '15 at 6:25

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    $\begingroup$ If Perl Harbor wasn't destroyed they probably would have bombed Python Bay. $\endgroup$ – Samuel Dec 3 '15 at 18:27
  • $\begingroup$ As long as Java still surrenders, I'm good with that! $\endgroup$ – Michael Broughton Dec 3 '15 at 18:57
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    $\begingroup$ It would probably delay our entry into war with Japan, but Japan would have gone on to invade what was an American protectorate, the Phillipines, and a treaty signatory, Australia, so we would have been in it eventually. Also, Japan was already planning to invade the Aleutian Islands, which were American territory at the time. We'd still have entered the European war, since Germany declared war on the U.S. and we were obligated to England by treaty. $\endgroup$ – Howard Miller Dec 3 '15 at 21:42
  • $\begingroup$ @Brythan, no the other mention the fleet before actually attacking. So the attack also never happened. $\endgroup$ – clem steredenn Dec 4 '15 at 6:25
  • $\begingroup$ @bilbo_pingouin Sorry, you're correct that the attack never happened, but this is still different. In that one, the Japanese fleet is destroyed or crippled. In this one, the fleet is intact and able to hit other targets. $\endgroup$ – Brythan Dec 4 '15 at 14:33
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If the fleet sailed to Pearl Harbor than the Japanese admiral would have attacked. To avoid Pearl Harbor Japan would need an oil source. Either the U.S. does not start an oil embargo on Japan or Japan decides to focus on taking away Russian oil fields instead of Pacific/U.S. oil fields.

  1. U.S. continues to sell oil. U.S. avoids any entry into the war. Japan mostly ignores Russia and keeps expanding in China and occupying the Dutch East Indies and British and French colonies. USSR eventually defeats Germany. But it takes longer. Most of Europe becomes socialist/communist but never reclaims its colonies from Japan. Three Super Powers emerge. USSR, Imperial Japan, and the U.S. The U.S. might be the least of these. After seeing the loss of life by others in WW II might even reinforce its pacifistic nature.

  2. U.S. cuts off oil and Japan focuses on Russia. Russian troops are better equipped than the Japanese. The Japanese suffer disproportionate losses and only make gradual gains with much loss of life. Their continual pressure forces Russia to keep troops in the east though. Valuable reinforcements are missing at key battles. Russia loses Stalingrad and the Caucasus. Their military machine is crippled without the extra oil. The Japanese focus on Russia eventually allows them to make headway and gain the Eastern oil fields as well. Russia is unlikely to surrender and continues fighting for years. Eventually turning into a guerilla war and terrorist strikes. The Axis powers win. However terrorism and separatist movements are likely to cause unrest for decades to come. Unless more mass genocides wipe them out. This rout makes it more likely the U.S. would enter the war. But they might only declare war on Germany. Without the galvanization of the Japanese sneak attack the sleeping bear of the U.S. economy might never awaken and it might be to little help to late. The U.S. would probably make peace once its allies were out of the war. But they would continue to foster the unrest and be a strong super power against the German and Japanese super powers.

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The loss of Pearl Harbor substantially crippled the US's naval forces. If we were able to know about--and therefore stop--the bombing, the war would have ended much more quickly and possibly without needing the Manhattan Project. The US would have had its full Pacific fleet with which to pressure Japan, vastly quickening the end of the war on that front. I suspect that the atomic bomb would still need to be used in order to cause Japan to surrender, as I don't think that the increased naval capacity would change much, but there's the slight possibility.

I don't recall if any of the Atlantic fleet was re-positioned to the Pacific theater due to the loss of Pearl Harbor, but if any ships were reassigned (I suspect at least a few were), then in this alternate timeline, those units would have been available to maintain their original orders.

Long story short, the war would have ended much more quickly, leading to all kinds of side effects, such as prisoners in concentration camps who died having survived (as the timeline moved up, causing the prison closure now being at a point in time prior to their execution), so it would be worth looking into any famous individuals who died in the final weeks of the war. Even just mentioning a name or two would be a neat little easter egg for those that recognize it, such as Kurt Gerron, even if it isn't relevant to the plot.

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    $\begingroup$ Disagree big time. Pearl Harbor decisively demonstrated the power of aircraft carriers, and without it the US would likely have produced more battleships (and fewer carriers) than they did. This would have reduced the effectiveness of the Pacific Fleet, and prolonged the war rather than shortened it. People (and countries) learn much more from defeats than victories. $\endgroup$ – WhatRoughBeast Dec 3 '15 at 19:45
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    $\begingroup$ The obvious presumption here is that the US would enter the war at about the same time, though in our timeline Pearl Harbor was what finally sparked the US entering the war. I don't necessarily think that's a bad presumption, but maybe if you spoke to how soon you expect the US to enter in absence of Pearl Harbor it would influence how much of an affect their increased fleet size would have on acelerating the war. $\endgroup$ – dsollen Dec 3 '15 at 19:53
  • $\begingroup$ I vaguely recall hearing that the US was not committed to defeating Japan near the beginning of the war. If my memory is correct they faught mostly a stalling game with Japanese fleets while committing their main forces to Germany because they felt they could take Germany much sooner which would ensure eventual victory. Assuming I'm even correct in that understanding what would that mean with an increase fleet? are you thinking they would focus on a more offense fight with japan since they had more numbers, or still focus on Germany? $\endgroup$ – dsollen Dec 3 '15 at 19:57
  • $\begingroup$ @WhatRoughBeast I think that's a very interesting point. However, wouldn't the US have lots of opportunities to be defeated by aircraft during the war? I would imagine we would take a beating from them early on and switch over to aircraft almost as fast right? Still, that also points out the fact that Perl Harbor fleet lacked air craft carriers. Even if more of the Fleet survived perhaps it wouldn't be that damaging without aircraft support or a lesson in the importance of such. It may get mangled going up against carriers soon afterwards, teaching us the importance of planes $\endgroup$ – dsollen Dec 3 '15 at 20:03
  • $\begingroup$ @dsollen - We'd have learned eventually, but it would have been longer process than you think. Military officers tend to be very conservative, in part because their formative years are spent learning that a certain way of doing things is necessary for survival, and this conditioning can be hard to cast aside. The Pacific Fleet did include 3 carriers, but they were out on patrol. And please, it's "Pearl" with an "a". $\endgroup$ – WhatRoughBeast Dec 3 '15 at 22:11
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I'm not sure if the scenario you paint could have happened. Japan felt it needed to knock the United States out of contention in the Pacific in order to break the embargo and secure sources of raw materials and labour for the Co Prosperity Sphere and the Empire. Even if the US fleet has steamed out of Pearl Harbour the Japanese may have felt compelled to attack, to deny the US the forward staging area and fleet base, essentially hoping the Pacific Fleet would have to steam back to the West Coast or die on the vine. (The Japanese were apparently unaware that the USN had been conducting experiments with fleet replenishment under weigh, and American Admirals were quite quick to suggest that this was the real key to the American victory in the Pacific).

Given the entire Japanese naval strategy rested on the idea that the US Navy was quickly knocked out of the war, and making the (unwarranted) assumption that American industry wold not be able to replace the fleet in any reasonable period of time, they might have chosen to attack the base and destroy the fuel and facilities based there, then steam on seeking to engage the US battle fleet before it could escape to safety.

The only way to avoid this branch of history would be to scroll back further into the 1930's where the Imperial Army and Imperial Navy were engaged in a power struggle over which direction the Empire should expand. The Imperial Army argued that taking Siberia would secure a rich resource base, secure from both attack and close enough to the home islands that transporting the materials would be easy and relatively safe. The Imperial Navy argued that the European Empires had already developed resources in SE Asia, and there was an existing pool of labour available to extract these resources for the Co Prosperity Sphere. The Imperial Army had the upper hand until they were defeated by the Russian Army at the Battles of Khalkhyn Gol, when the Naval faction gained ascendency and pursued their "southern" strategy.

If Stalin had managed to purge all of Marshal Mikhail Nikolayevich Tukhachevskii's mechanized warfare acolytes in 1937 (including Georgy Zhukov), then it is quite possible that the Russians would not have been able to defeat the Japanese 6th Army, and the Imperial Army's "northern" strategy would continue to have been implemented, avoiding the need for the risky Pearl Harbour gambit.

The United States could continue to press the issue of the embargo, but lacking a Casus belli would not have moved into war (the American public and the Congress were generally opposed to war, regardless of what Roosevelt thought, and only the dramatic action of an actual attack against American forces during a time of "peace" was enough to inflame American public opinion for a war). Imperial Japan would have been able to break the embargo with more and more materials extracted from China and eventually Siberia, while gradually drawing the nations of SE Asia into the Co Prosperity Sphere through economic enticements rather than conquest.

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  • $\begingroup$ "if the US fleet has steamed out of Pearl Harbour the Japanese may have felt compelled to attack" Maybe, maybe not. Note that Naguma vetoed a third wave which would have targeted facilities and POL storage. As an old battleship admiral, he did not think in terms of aviation and the need for basing. He had fulfilled the explicit goals of the attack, with remarkably light losses, and that was good enough for him. $\endgroup$ – WhatRoughBeast Dec 3 '15 at 22:14
  • $\begingroup$ I like this answer very much. You seem quite informed and I appreciate that, but what I like most was that you were able to both point out the flawed presumption of my question and still suggest a situation where it would happen. I appreciate answers that both try to be realistic if the question is flawed but still try to meet the spirit of the question anyways! One thing I would ask, presume the scenario happened as you said, where the Japanese attack Perl Harbor despite it being defended what do you imagine the results being? you imply they would discover the fleet had.... $\endgroup$ – dsollen Dec 3 '15 at 22:22
  • $\begingroup$ auxiliary supply ships to refuel them that the Japanese didn't know about, but help me out on what that would mean for them? would they be facing a counter attack or simply see the US fleet get away to continue fighting in the future? $\endgroup$ – dsollen Dec 3 '15 at 22:24
  • $\begingroup$ They would not be facing a counter-attack. Those BBs were too slow to catch the Japanese fleet. $\endgroup$ – Joshua Dec 4 '15 at 0:29
  • $\begingroup$ If the Japanese had attacked Pearl Harbour after the fleet had steamed out, their losses would be somewhat heavier (the base would be on alert), but not much heavier since the heavy firepower of the ships would not be there to add to the defense. The Japanese didn't seem to appreciate the usefulness of supply under weigh, but *could *have caught up with the US fleet. The results would be hard to determine; the US fleet could prevail if the US aircraft carriers were present (much like Midway), or maybe the Japanese would have the edge due to their superior torpedoes. $\endgroup$ – Thucydides Dec 5 '15 at 3:34

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