~First, understand that it was not unprovoked. FDR wanted to go to war with Hitler, but he knew that neither Congress nor the American people would let him. He figured that if Japan could be provoked into attacking, overt US participation in the war in Europe would follow. (From the various "Neutrality" Acts through Cash and Carry to Boats for Bases to Lend Lease, FDR was increasing US participation by the day and he all but declared war on Germany during his 9/11/41 Fireside Chat when he announced that he had ordered US naval ships to fire on German warships on sight and without provocation whenever they were spotted in US "defensive waters", which he described as the entire North Atlantic, especially from US ports along the coast of Canada, past Greenland and Iceland to the shores of the British Isles and near the approaches to the Mediterranean. He transferred several ships from the Pacific Fleet to the Atlantic Command to engage in hunter killer missions he called "neutrality patrols" to hunt down and sink German warships.
At the same time, he was beefing up bases across the Pacific which threatened vital Japanese shipping lanes - places like Pearl, Midway, Wake and the Philippines. He created an entire new army, the USAFFE and based it in the Philippines, bring Douglas MacArthur out of retirement to lead it, then he created the FEAF, the largest collection of US warplanes outside the US to give it air support. On the same day he created the USAFFE, he ordered the seizure of all Japanese assets in the US. This on top of the oil, steel and rubber embargoes. Concurrently, he was sending all kinds of aid, including unofficial military "advisors" to our then allies Mao Zedong and Ho Chi Minh to help them in their wars against the Japanese. (When the US double-crossed Ho at Tehran, Tokyo Bay and again in 1946, 1954 and 1956, the Vietnam War was the inevitable result, but there's no room to go into that and it is far beyond the scope of your question). The McCollum Memo is but one blueprint by which the Japanese attack could be provoked.
FDR ordered the Pacific Fleet moved from the safety of San Diego to Pearl. CinCPac Adm James Otto Richardson disobeyed the order several times, warning that the move was an unnecessary provocation likely to induce an attack and that the fleet would be defenseless against a carrier based air attack, as US wargames had proven (and FDR knew by not later than January 1941 that the Japanese were practicing just such and attack). Richardson obeyed his orders, but continued to express his concerns. He was relieved. Chester Nimitz refused the post because he shared Richardson's concerns and he knew when the attack came and the fleet was destroyed, CinCPac would be the fall guy. Husband Kimmel, although he agreed with Richardson and Nimitz, took the job and, as predicted, got the blame.
Japan did not want war with the US. They knew they could not win once the industrial might of the US was unleashed against them. When they received the Hull Note, they viewed it as a declaration of war and they believed they had no choice but to try to take out the US Naval presence from their sphere of influence in one massive stroke, thus forcing the US to the peace table, releasing Japanese assets, removing US bases from their back yard, easing the embargoes and otherwise treating Japan as an equal in the community of nations rather than as a pawn and a subjugated nation. Had the carriers been in port (Enterprise should have been but was delayed by weather - she was close enough that her fighters did see action on 12/7), had Genda been allowed to launch the third wave against the sub pens, oil storage facilities, maintenance yards and drydocks or had Nagumo been allowed to take out Midway on the way home, the plan might have worked.
War between Germany and the US was inevitable once FDR got his war in the Pacific and Hitler knew it. Not that it mattered. The US and UK had very little to do with the fall of the Third Reich. That war was fought and decided on the Eastern Front, between the Wehrmacht and the Red Army. The Red Army sustained and caused 90?f the casualties in the war in Europe. The Normandy Invasions was a minor skirmish against third rate, outnumbered, under-equipped and poorly trained remnants of the Heer Army. Rommel and von Rundstedt had fewer than 1 million troops with which to occupy and defend all of Western Europe (and were able to commit maybe 1/4 of them at Normandy). By contrast, in the East battles between armies 1 million strong were regularly contested in the East, often at the same time. When US troops first saw action in North Africa against the Vichy French and Italians, Army Group Central had been destroyed at Moscow, Army Group South had ceased to exist at Stalingrad and after Kursk and Smolensk and Army Group North was stalemated at Leningrad and about to be quarantined and taken out of the war on the Courland Peninsula.
Answered By: Oscar Himpflewitz - 2/21/2012