The amazing Captain Den Beste typed an amazing response to John's question. I'm posting it here in full because it's so great. Really, Steven ought to put it on his site.
Please excuse my instinct to think of all of you as my fellow Americans, as I lapse into "we" when it probably should be "the US"
My main question being, What was Japan's motivation from bringing the US into W.W.II? I know a couple of factoids, and have a few theories but I cannot remember ever going very far into the Pacific war in school. It always seemed to be enough to say the US was in the war because the Japanese attacked. No one ever went into why they might have done so.
1. I know American pilots were flying American planes painted with Chinese colors, so we were already fighting a secret war against Japan.
2. I am suspecting it might have something to do with the forced opening of Japanese ports those many years before, but I don't know.
3. Did it have anything to do with Japan's becoming industrialized, but not having much in the way of raw materials? Did the US impede this in anyway, were we competing for colonies?
4. Were the Japanese just keeping a bargain with their allies to keep the US from being too helpful to Europe?
5. If that is the case, (Here is where the change up kicks in) Why did Hitler leap at the chance to declare war on the US after the attack? To divide our forces? Or to legally sink our ships?
There was more, but I cannot remember it now. Anybody have any answers? I am going to post this on Rob's list too, as I think SDB might be my best hope for an answer. Please forgive me.
I know the Japanese were particularly brutal, and that Lia might have relatives who were involved in that theater during the war, so I hope I have not made it seem like I am trying to excuse Japan's actions. I am really surprised that everything I have been taught has been so one sided. With Germany there is plenty of talk about the events that lead up to what came to pass.
Then as now, Japan's industry wasn't sustainable with local resources. Then as now, Japan depended primarily on imports of raw materials, and there were important ones where the Japanese didn't control their sources of supply. The seeds of the conflict between Japan and the US stretch back a long way; in most accounts it begins with the visit by Commodore Perry in 1853.
Leaving out a lot of context, what you end up with by the 1920's is a government in Japan where the Army is aggressive and imperialistic and essentially in control. You also have an army where mid-level officers have far more influence than they really ought to. Japan already controls Korea by this point and is looking fondly at Manchuria, and some officers in that area manufacture an incident and go into full scale attack without permission of the civilian head of government. Before they know what is happening, the army has already invaded large parts of Manchuria and is moving forward. (Many historians actually date the beginning of WWII to this incident in 1931, instead of the more usual dating based on the invasion of Poland.)
This eventually lead to war with China. Now China then was an ally of the US, and though Roosevelt's military situation was weak his economic and diplomatic power was considerable. There were threats and counter threats, and the US started imposing trade sanctions against Japan.
The final straw was imposition by the US, UK and in particular the Dutch of an embargo on petroleum and scrap steel sales to Japan. This threatened to stop Japanese industry and also to immobilize Japan's fleet.
They had enough petroleum stockpiled for perhaps 18 months of operation and after that they would be stuck. They needed the Indonesian oil fields (the best source in that part of the world, at that time under Dutch control).
So here's the decision that Japan was facing. It had, in 90 years, moved from being a backwater which had been shamed by a handful of American steamships in 1853 to a fully modern industrialized country with a formidable navy, something no other third world nation had done in that time. It could back down, apologize, stop the war in China, and slink back home with its tail between its legs. Or it could shove all its chips into the center of the table and attack.
Retreat was impossible. If such an order had been sent to China it would have been ignored, and the likelihood is that leaders giving such orders would have been assassinated. (In the 1920's, Japan's government was known elsewhere as "government by assassination"; it was preposterously common for mid-level army officers to order the deaths of politicians they didn't like.)
There were cultural issues here, too; the Japanese had abolished the caste system but still believed that their men couldn't be defeated. They also believed that the US was soft, effete, bourgeois, and had no real stomach for war.
On the side of the balance favoring attack was the fact that the Japanese naval Air Force was the best in the world at that time (though no-one outside Japan knew it). The US is weak "as we all know" and won't be willing to take a lot of losses "as we all know" and if Japan could take a lot of territory fast and then reinforce it, the US would decide it wasn't worth the price to take it back. And "as we all know" the US would lose as many men in the fight as Japan would. So the US would bargain rather than fight "as we all know".
At Gettysburg Colonel Chamberlain defended Little Round Top and was faced at one point with either retreating or attacking -- and since the consequences of retreat (loss of the entire battle) were intolerable, he ordered an attack. It was a brilliant (and desperate) move and succeeded beyond his wildest hopes. Many consider that the single critical battlefield decision of the entire battle (and indeed of the entire war since that was the turning point).
The Japanese made the same decision. They did, I think, understand that it was risky but considered the risk acceptable given the intolerable consequences of not attacking. What they didn't perhaps understand is the degree of fury that a sneak attack on Pearl Harbor would raise in the US. And some leaders in Japan (particularly Admiral Yamamoto) understood the industrial capacity of the US but none really understood the degree to which it would be trained on Japan, like a cannon aimed at a flea. Ultimately the war in the Pacific wasn't US soldiers against Japanese soldiers, or US ships against Japanese ships, but rather US industry against Japanese industry -- and that wasn't a contest. Within two years, from scratch, the US already had created a more powerful fleet and a more powerful airforce in the Pacific than the Japanese had managed to create in the pre-war years. And it only got worse from there. By 1945 the US fleet and air power in the Pacific was frankly incredible.
Another major mistake the Japanese made was to massively underestimate the importance of signal security. They believed that the Japanese language itself was sufficiently obscure and difficult that the Americans would never be able to understand their code transmissions. Again, they underestimated both the US and UK, and in the course of the war I'm not aware of a single Japanese cipher which resisted attack. On the other hand, the Japanese attempts to break into US code were total futility, and such efforts were always badly understaffed. By the end of the war, the US had at least a thousand times as many men involved in signal intelligence as did the Japanese, and this was a major contributor to the conduct of the war. Midway is the example most people know about, but it was in fact not the most important. Vastly more important was that the US was reading the ciphers used by the Japanese to control their merchant shipping, and using that information to control its submarines to attack said shipping. After 18 months of truly embarassing torpedo malfunctions, by mid 1943 the US submarine force began to leave a bloody swath through the Japanese merchant marine. The US succeeded in doing to Japan what the Germans failed to do to the UK: starve it out by submarine blockade.
The Flying Tigers never were an issue, by the way. The final straw which lead to war was the embargo on oil and scrap steel, which the Japanese took as an aggressive act. To this day you'll find Japanese who think that Japan really had no choice but to attack, and honestly think of Pearl Harbor as a defensive move forced on them by the US.
Also, there was never really an issue relating to obligations to Germany. That "alliance" was never really an alliance in the sense that the ones between the US, UK and USSR were, where each made sacrifices for the other and where they coordinated their efforts (such as the USSR's offensive launched in June 1944 to coincide with the Normandy invasion).
A demonstration of the fundamental unimportance of the Japanese/German alliance is that at the time of Pearl Harbor and for the rest of the war, Germany was locked in a death struggle with the USSR. Hitler hoped that the Japanese would attack the USSR in Siberia, draining away resources from the German front. Yet Japan never attacked the USSR, maintaining an uneasy truce at the border of Manchukuo and Siberia. (This was shattered in July 1945 when the USSR attacked.) This is because the Japanese had gotten their asses kicked by the USSR in 1939 in a brief war there. After Pearl Harbor, Germany (and then Italy) declared war on the US (much to the relief of both Churchill and Roosevelt, who wanted the US to fight Germany and had to figure out a way to convince the US public that an attack by Japan should result in a US war against Germany). A lot of people have wondered why Hitler did this; it turns out that he did so because he was hoping that Japan would in turn declare war on the USSR -- which never happened.