I am well aware the Japanese invaded Dutch Indonesia, but I don't understand for the life of my why they thought it a good idea to pre-emptively strike at the United States. I also know the US cut off its primary oil supply, and Indonesia was the closest and largest pool to replace it.
I have searched this on Reddit and Wikipedia and the usual answer is the Japanese feared FDR would declare war should they invade Dutch Indonesia, considering his naked hostilities towards them and their fellow axis members in Europe. As such, they tried to deal a crippling blow to the sleeping giant before aggressively expanding their empire in the Pacific.
Public opinion in the United States, however, was against going to war. Unless I'm mistaken (and please tell me if I am), Roosevelt only gained the support to go to war due to Pearl Harbor (no declaration needed from the US), and then a further declaration of war by Hitler allowed Roosevelt to enter the war in Europe without a messy senate vote. Roosevelt's 1940 platform even included a promise not to go to war. The argument that the US would respond to Japanese aggression in the Pacific with a declaration of war makes no sense. If the public was against getting involved in the nasty affairs of Europe, they certainly wouldn't want to get into a war over those European country's colonies. I can't imagine the people who don't want to fight Germany to free the Netherlands suddenly writing their congressmen to support Dutch Indonesia against the Japanese.
Did Japan just not realize how isolationist a good percentage of the country and it's elected officials were feeling? Could they have gotten away with just conquering Indonesia, with Roosevelt left with only paper-thin proclamations?
The main reason (amongst many) would appear to simply be resources and Japan's lack of it.
Japan has extremely little in the way of natural resources and was keen to extend its borders in Asia to get some (from WW1 as part of the peace treaty for example they took the German colonies in that theater). A key region for Japan was Manchuria and wider China, they conducted many offensives in the area. This caused several issues with the Western powers, in particular the USA as they traded heavily with China and Japan was keen to isolate and starve out China. As a retaliation the USA ceased trading armaments such as ammo and planes with Japan in 1940.
What followed was an arms race of sorts, the USA built up the Pacific fleet in Pearl Harbour and Guam and Roosevelt made it clear that if Japan aggressively expanded by attacking its neighbours the USA would intervene.
The USA then stopped trading oil with Japan. This was a big issue for an economy that wanted to expand through militarization. Japan had to look to Dutch and French colonies (such as Indonesia) as well as British held Singapore. For Singapore in particular the Japanese command intended to attack but believed that this would cause the USA to declare war on Japan.
Japan needed the resources, desperately, but couldnt fight the entire Pacific fleet. The Japanese command felt that their objectives would ine inevitably bring the USA into the war so conducted the "pre emptive" strike on Pearl Harbour, Guam and other US bases with the aim of then having naval dominance in the Pacific. The aim was never to fully "defeat" the USA in a traditional sense at this point, but dominance in the Pacific would allow them to expand in Asia without fear of a US attack.
From the US stand point throughout much of this period they wanted to avoid war but discourage Japanese aggression. To this end they were actually prepared to abandon the entire Phillipines rather than defend it should conflict break out there (this never happened in the end). The actions of Japan led to trade sanctions which led to Japan almost doubling down on the aggressive expansion.
I hope this helps, my particular background is more political than military so I have probably missed a fair bit from a military historians point of view.
This was the plan from the start as Japan set its sight onto the Southern Pacific especially British Malaya and the Dutch East Indies. The British and Dutch were also more focused on the Germans and Italians in Europe and would take time to respond to any threat in the Pacific. Their resource problem would also be dealt with after seizing the resource rich areas in the Dutch East Indies and British Malaya.
Japan’s plan was to take the territory and then fortify their hold on the South Pacific. While the European forces in this area would be isolated from the home countries and any help would take time to cross the ocean. The Japanese would establish naval superiority in the area and basically cut off any aid or reinforcements to the troops. However there was one flaw in this plan: the US Pacific fleet. While the US public was divided on another war, if the Japanese moved against the British and Dutch the US could be next. The US government could declare war on Japan as seeing it as a war of aggression by the Japanese after trying to negotiate a peaceful way of ending the expansion into Asia. The US would then mobilize it’s fleet and industry to the fight the Japanese. The US would not just look away while the British and Dutch were invaded in their own backyard. If this would happen, the Japanese would stand no chance against US. The Japanese were not idiots, they knew the Pacific fleet was a threat and any moves on the South Pacific would be seen as an act of war by the US. The attack on Pearl Harbor was to neutralize the immediate threat and then establish control on the Pacific. The plan wouldn’t have differed much if they decided to ignore the US for whatever reason. Even if they decided to hope that the US public would prevent the US retaliating, sooner or later they would join in and Japan stood no chance. And then if the Pacific fleet moved out, there would be no way the Japanese could destroy it or have an opportunity as good as they did in Pearl Harbor. This would become a war of attrition and one the Japanese could not win.
Even if they decided to ignore the US fleet for now, they would eventually join the war and any chance to cripple it would be welcomed. Better to strike while the Pacific fleet is docked and easy prey then to fight it in pitched battle and risk their own ships. Though it’s plan of just hoping the US wouldn’t join and would be ok with holding all that territory was a fool’s wish. Even after the attack on Pearl Harbor, this was the plan the Japanese went with and there was no way for the Japanese to win against the US who was superior in every single way: in industry, population, and military strength.