Why did Japan attack Pearl Harbor? What did they hope to accomplish? It seems like just about the stupidest tactical decision they could have made. Help me understand.
Bonus Question: How did the Nazis react to news of the attack? Did Hitler know about it in advance?
You're asking a question on two levels here: Why did the Japanese attack, and why did they attack Pearl Harbor?
As to the first question, the U.S. Army's Strategic Studies Institute says it best: The key resource issue for Japan was oil. In 1941, Japan relied upon the United States for 80 percent of its oil. When the U.S. embargoed Japan, this put Japanese decision-makers in a bind.
The Japanese military -- its Army and Navy both -- were heavily dependent upon oil. With the start of the embargo, the clock started ticking for Japan. Each day that passed meant its reserves were that much thinner. It could either withdraw from the war in China or it could seek oil elsewhere.
Siberia was attractive, but it would take years to develop the industry needed to tap Siberian oil resources. The Dutch East Indies were already producing large amounts of oil, and if they could be seized, they could be exploited immediately.
Moreover, the Dutch, unlike the Soviets, were militarily weak. The occupation of the Netherlands meant the East Indies were on their own, militarily.
There was a problem, however: Japan knew that the United States and Britain would not sit idly by as it declared war upon and occupied the East Indies. Britain and the United States occupied strategic points along the hypothetical supply line between the Japanese Home Islands and the East Indies. From their bases in Singapore and Hong Kong (British) and in the Philippines (the U.S.), Britain and the United States could blockade or otherwise halt shipments of oil from the East Indies.
This meant that not only did Japan need to act quickly, it needed to act against the United States, Great Britain and their allies all at once.
This brings up the second question: Why Pearl Harbor?
Well, that can be credited to the U.S. Navy and Japanese admiral Isoruku Yamamoto, commander in chief of the Imperial Japanese Combined Fleet. I'm going to draw much of this second answer from John A. Adams' If Mahan Ran the Great Pacific War: An Analysis of World War II Naval Strategy, but I'll include references to other works where appropriate.
For decades before the outbreak of war, Japanese plans had called for elements of the Imperial Japanese Navy to operate against other targets (presumably to the south or north) while the main body of the Japanese fleet was held back for a main battle against the responding American fleet. Lighter units would attrit the American fleet as it advanced toward the Japanese fleet and the "decisive battle." (American naval war plans, notably Plan Orange of the Rainbow war plans, envisioned a coordinated move westward, precisely as the Japanese expected, but this approach was largely abandoned in the later half of the 1930s.)
Through the 1930s, the location of this climactic battle changed as Japanese technology and infrastructure advanced. As air bases were developed among the Pacific islands and the Japanese Navy developed its carrier arm, the expected grand encounter was placed farther and farther east. By 1938, it was expected to take place near the Marianas Islands. Regardless of the location, one thing stayed constant: The Japanese battle fleet would remain close to the Home Islands and await attack. (See Goldstein and Dillon's The Pearl Harbor Papers: Inside the Japanese Plans, Prange's At Dawn We Slept or Evans and Peattie's Kaigun for more on Japanese plans)
In May 1940, the U.S. Navy transferred its Pacific Fleet headquarters to Hawaii from San Diego. This move had a two-fold objective: It was a de facto warning against Japanese military aggression in Asia, and it put the U.S. fleet into better position for operations in the western Pacific, should war break out.
A few months later, the U.S. Congress approved the Two-Ocean Navy Act, the largest naval procurement bill in U.S. history. Included in the bill were $8.55 billion in appropriations for 257 major combat ships and 15,000 aircraft, the approximate equivalent of two Imperial Japanese Navies. At the time, it was estimated that the Act would take five to six years to complete.
In Japan, Yamamoto was aware of the U.S. Navy's construction program and its abandonment of Plan Orange. For these reasons, Yamamoto became convinced that Japan had to win a war against the United States quickly. In the long run, he theorized, the United States would overwhelm Japan in a war of attrition. "Japan must deal the U.S. Navy a fatal blow at the outset of the war," he wrote a fellow admiral, long before Pearl Harbor planning began.
As early as April 1940, Yamamoto told Vice Admiral Fukudome Shigeru to order Rear Admiral Onishi Takijiro to study a Pearl Harbor attack under utmost secrecy. Later, after learning of the British carrier attack on Taranto, Italy in November 1940, Yamamoto became convinced that an attack on Pearl Harbor was the correct approach.
At the start of 1941, Yamamoto officially threw out Japan's traditional "defensive-reactive" strategy, but not without plenty of opposition from the Naval General Staff, which was in charge of drafting naval strategy. Yamamoto also reorganized Japan's carrier force. Until this point, Japan had followed Western navies by organizing its carriers into pairs or individuals backed by escorts. These operated in scouting or raiding roles. Yamamoto overturned this idea by — in April 1941 — combining all eight Japanese carriers (including those still fitting out) into the First Air Fleet. This gave him the ability to throw almost 400 strike aircraft at a single target.
That this target should be Pearl Harbor was not obvious to all. Admiral Osami Nagano, chief of the Imperial Japanese Navy General Staff, was in favor of a plan that focused the Japanese fleet's striking arm directly upon the oil of the Dutch East Indies. Yamamoto disagreed wholeheartedly: "If we have war with the United States, we will have no hope of winning unless the United States Fleet in Hawaiian waters can be destroyed," he said.
All through 1941, Yamamoto argued his case. If the American fleet was destroyed, he contended, the East Indies would fall into Japanese hands — there would be no force able to stop it. If the fleet was not dealt with first, it might be the Japanese fleet that was caught by surprise. Attacking the East Indies would require many separate attacks, and the American fleet -- already concentrated in Hawaii -- might defeat the Japanese fleet in detail.
In fall, Yamamoto played his trump card -- he threatened to resign unless he got his way. Finally, on Nov. 3, 1941, Nagano agreed to the attack.
Yamamoto's goal was to knock out the American fleet at Pearl Harbor, then fend off American counterattacks long enough for the American public to grow tired of the war and accede to a peace that left Japan in control of the oil coming from the Dutch East Indies.
This strategy died because of a political miscalculation: When Japan failed to deliver its declaration of war before the Pearl Harbor attack, Yamamoto knew the mistake meant the American public would not grow tired of war until the "treachery" was avenged. The 1970 movie Tora! Tora! Tora! summarized this, when it attributed a quote to Yamamoto that may have been made up on the spot but was still true: "I fear all we have done is to awaken a sleeping giant and fill him with a terrible resolve."