From what I've read it seemed like most Americans were opposed to entering the war in Europe. It seems like eventually the US would have had to enter the war, but what was Japan's motivation for an unprovoked attack on the US? Ive also read some stuff saying that the US knew the attack was coming and allowed it to happen to stir up nationalism and pro war sentiment, like the sinking of the Lusitania in WW1. (Not saying the US allowed that to happen, but that event gave way to the US joining the war.)
A minority faction in the government pushed through the attack because all other factions had failed.
In the year 1898, naval officer Akiyama Saneyuki travelled to the United States and discovered a new American innovation in naval warfare - wargames. Despite being called games, these stressful, real-time, probability based war simulations were not fun, but were designed to predict the outcome of battles accurately with the help of statistics. This allowed navies to rehearse wars and battles before they happened, and proved decisive in the Russo-Japanese war.
The year is 1905. The Russo-Japanese war comes to an end and President Theodore Roosevelt brokers a peace when Japan is on the verge of financial collapse - a fact that no one but the Japanese government is aware of. The peace terms, as a result, are moderate. This angers the Japanese people, who accuse the US of a betrayal. Something else important happens during this war - Japan sinks almost the entire battleship row of the once mighty Russian navy. As a result, the only navy that can challenge Japan in Asia is the US navy. As a result, all of Japan's prized naval wargames, from now on until 1941, have the US as the simulated enemy. Over time, Japanese naval officers get so used to pretend fighting the US that they simply assume the US is the enemy.
By 1918, World War I is over, but the next years bring new crisis to Japan. During the war, Japan's economy heavily integrated with those of the West, as Japan provided goods to war-torn Allied economies. Japan was now dependent on external trade. At Versailles, Japan is awarded former German colonies in China, but the dispute persists. Japan concludes the secret Nishihara Loans with Premier Duan Qirui of China, and forces the Chinese government to agree to a majority of the oppressive "21 demands", which, if all agreed to, would reduce China to a trading dependency of Japan. Once the secret arrangements between Duan Qirui and Japan are discovered, protests break out throughout China, and Duan is deposed. The US steps in to mediate, concerned that its "Open Door" policy in China - stating that China is a free for all market for all Great Powers, and should be no one's exclusive trade zone - is in danger. In 1922, Japan and China agree to a doublethink solution in which China officially gets some of the German concessions, but Japan actually runs them.
The 20s are a time of prosperity, growth, and cooperation by Japan. Aggressive policies from 1919-21 brought Western ire, so civilian cabinets, made supported mainly by the upper middle and upper classes, seek reconciliation. They sign the Washington Naval Treaty, in which Japan is entitled to 3/5 of the US Navy's battleship row. As Isoroku Yamamoto points out, this is in favor of the far less industrialized Japan, which only needs to focus on Asia, while the US Navy needs to split itself between two oceans. Most Japanese, however, lack the foresight of Yamamoto, and the treaty is unpopular. Also in the 20s, Japan becomes increasingly dependent on American oil.
Late 1931. The Great Depression is in full swing. Britain, France, and the United States start forming "trade zones" with their empires and allies, increasing trade barriers and locking competitors without spheres of influence - like Germany, Italy, and Japan - out of much of the world marketplace. Ishiwara Kanji, an officer assigned to guard Japanese railways in Manchuria, believes Japan is suffering from overpopulation, lack of resources, and the lack of a trade zone, and unilaterally orders his men to create an incident with the Chinese. Rogue officers order an all-out invasion of Manchuria, which succeeds, forcing the Tokyo government to accept their soldiers' fait accompli and create the puppet state of Manchukuo, stealing 90% of China's industrial base. The League of Nations finds Tokyo at fault, and as a result, Lord Matsuoka leads a Japanese walkout from the League.
In the 30s, the military is rife with factions. The Imperial Way Faction believes that the spirit of the Japanese soldier must be ever strengthened, so that a self-sacrificing spirit may prevail against all enemies. The Control Faction believes that only modern equipment and tanks will win wars. A failed coup by Imperial Way sympathizers settles the intra-army argument in favor of Control. However, there are disputes between branches. The army believes Japan should expand into the Soviet Union, which it sees as the Africa of Europe - still believing the country to be backwards and under-developed. They call this the "Northern Road". The navy, in contrast, assumes the US is Japan's enemy by virtue of simulating so many wargames against it. They believe that knocking out the US fleet and invading European colonies in Asia is the best route, known as the "Southern Road". In the navy, literature forecasting a race war in which the US leads the White Races and Japan leads the Asian Races proliferates.
In the late 30s, the Soviet Union settles the Army-Navy dispute for the Japanese. A series of border conflicts end in decisive Soviet victory. Stalin's five year plans massively industrialized the nation and mechanized its army - it becomes clear to Japanese military planners that a ground war with the USSR would be a disaster. Under Inspector General Hideki Tojo, the army supports the navy position.
There is one more obstacle in the path of the navy's plan to start their new war - civilians. Lord Fumimaro Konoye rises to power in the mid-30s, and takes Japan on a wild adventure in China which is part of neither the army nor the navy's original plans. Believing the Chiang Kai Shek regime to be weak, he invades after a clash of arms, assuming that Chiang will rapidly cave and agree to be a dependency. He is wrong, and Japan begins a costly occupation of half of China's population centers.
This war has the added effect of ruining Japan's remaining relations with the US. Americans are horrified at Japanese war crimes in China, and the US government issues increasing levels of sanctions against Japan. Konoye, aware that war would mean the end of his regime, desperately tries to make peace in 1940-41. Preparing for the worst, he dispatches Matsuoka to the USSR, successfully concluding a non-aggression pact. Tojo pressures the Emperor to give Matsuoka a deadline to come to a settlement with the US. After much factional infighting, US policymakers send an ultimatum to Japan demanding total withdrawal from China, which was politically unthinkable. Konoye and Matsuoka fail to overturn this by the deadline, and so the Emperor replaces Konoye with Tojo. The navy is free to go down the "Southern Road". The US basically makes this necessary, as its final round of sanctions cut off Japan's oil supply.
Within the navy, there are two approaches for war with the US. The first envisions a US "charge" to save the Philippines - Japan ought to attack this "charging" fleet and whittle it down. The second, championed by Isoroku Yamamoto, envisions an aerial surprise attack, destroying the enemy battleship row.
As you mentioned, Pearl Harbor seemed stupid in hindsight, but with the theories of the time, it made perfect sense. The IJN read the theories of Mahan, which dictated that, because ships took a long time to build, that destroying an enemy's battleship row meant victory. The IJN believed in Mahan's idea that single, decisive engagements led to the end of wars. Further, the IJN was buoyed by confidence. Japan, from the rise of the modernists in 1868 to Pearl Harbor, had never lost a war. Always, the Japanese fought outnumbered against their foreign enemies, but won through superior doctrine and morale.
In the internal navy debates, Yamamoto & Co. get their way. Thus, Pearl Harbor.
War in Asia was not inevitable, and how the war occured was not either. A minority of naval planners, favoring a surprise carrier attack, got the military and political establishment on board because every other option had either been discredited or failed.
Sources:
Pike, Francis. The Development of a Death Cult in 1930s Japan.
Asada, Sadao. From Washington to London: The imperial Japanese Navy and the politics of naval limitation, 1921–1930.
Toland, John. The Rising Sun: The Decline and Fall of the Japanese Empire, 1936-1945.
Borgstrom, Jinna. Rhetoric of the Manchurian Crisis.
Lu, David. Agony of Choice: Matsuoka Yosuke and the Rise and Fall of the Japanese Empire, 1880-1946.
Iriye, Akira. The Origins of the Second World War in Asia and the Pacific.
Op you may also be interested this old answer of mine.
It specifically address long running conspiracy theories that either FDR had far more knowledge than he let on about the impending attack on Pearl vs just general war clouds gathering. Or that he specifically set the Pacific Fleet out there in Hawaii to act as bait for a war he could not otherwise get the US into.
Lets just say I find fault with both
Happy to answer followups!