Considering today is December 7th, I'll talk about why Japan attacked Pearl Harbor and why it pretty much signaled that Japan would lose the war with the US before it really even started.
So why attack the U.S. in 1941 when everyone knew that the US had an industrial output 10 times that of Japan? Well the Japanese had a lot of assumptions in their heads when thinking about war with the US. The first one being that war was inevitable between the Japanese and the United States considering the imperialistic conquering they had been doing around the Philippines (A US held territory). Another was that the longer the Japanese waited, the less they would be capable of waging war and the more capable the US would be at waging war. Due to this it made sense to strike as soon as possible because the Japanese navy was at its peak in 1941, the US oil embargo meant they didnt have long until they ran out of oil, and that as of 1941 Japanese naval tonnage was at a competitive percentage to the US. Another assumption was that a drawn out war with the United States would be almost impossible for the Japanese to win considering that everyone in the IJN and IJA knew that there was no way Japan could compete with American industry and their vast resources. The last assumption being that the only way to win this war would be through a strong defense and the sheer superiority of the Japanese race and spirit. The Japanese wanted to seize all territory in the south and central Pacific and fortify it, so that an American offensive through these fortified holdings would be so difficult and costly for the Americans that the loss of life would outweigh victory. They knew the US would win eventually but wanted to break the spirit of the American people and pull out of the war. This mainly comes from the Japanese thinking the US was filled with a bunch of greedy pacifists only concerned with material wealth and they would be scared of Japan, a nation of hardy warriors.
So all this thinking leads to the decision to attack Pearl Harbor and engage in all out war with the United States. The goal of Pearl Harbor and the subsequent invasions of the Philippines, Burma, and other European colonial holdings is to expel these foreign influences, make the world recognize Japan's Empire and delay US naval forces. With specific regards to Pearl Harbor, Marshal Admiral of the IJN Yamamoto wants pretty much one thing, to delay the US Pacific Fleet for at least 6 months so that Japan can conquer without fear in Southeast Asia.
So why does Pearl Harbor pretty much spell the end for the Japanese? Well, before the attack Japanese recon and intelligence reported that there were 3 aircraft carriers at Pearl Harbor plus the other ships that were there as well. The aircraft carriers however were not there on the morning of the attack and thus were not able to be damaged. This is significant because as both sides will find out in the coming months the most powerful weapon on the naval battlefield is the aircraft and thus aircraft carriers are a huge force multiplier. Also during the attack Japanese air forces were instructed not to hit the dry docks, only the ships. This was a pretty stupid decision considering the ships they only damaged could easily be repaired right after in the untouched dry docks. Lastly and probably most significant was that this sneak attack would pretty much put the entire American population into a yearning for Japanese blood. No matter how much pacifism or isolationism the American population and Congress wanted it was all out the window now and the only thing the nation wanted was complete unconditional surrender.
This would come back to bite the Japanese in the ass where those missed carriers, those easily repaired ships, and the now legit racial hatred the Americans had for the Japanese would come back to reek irreplaceable damage on the Imperial Japanese Navy at the Battle of Coral Sea and the Battle of Midway. Midway being of particular importance considering it is really the end of any hopes of military success the Japanese might have.
