Why did japan plan a large scale attack against the us pacific fleet at Pearl Harbor?

Answer 1

The short answer is because it was where most of the US pacific fleet was. By attacking it and crippling it, the Japanese military would be able to conquer other countries in Asia before the US could get involved.

The longer answer- Japan is a relatively small island group with little Natural Resources of its own. It had always had a fairly militaristic society centered around clan warfare and samurai and all that. Starting around the 1860's, Japan ended their samurai era and began quickly modernizing, with help from western countries like England, France and the USA (who hoped to exploit the weaker country for their own gain, as those countries were doing in China and India at the time). By 1910, Japan was more or less fully "modern", and had even fought and defeated major countries like Russia and China. They supported the Allies during World War I, and helped occupy German colonies in the Pacific.

But to continune modernizing, they needed resources like oil, rubber, and metal, which, as already stated, they had in short supply. Japan came to be controlled more and more by a group of extreme right-wing militarists who wanted their country to continue to grow in size and strength-- meaning they needed to conquer. So they started conquering during the late 1800's and their successes led to desire for more. They conquered Taiwan and Korea, and eventually began conquering large parts of China, especially Manchuria, where they set up a "puppet government" that they controlled.

In the 1930's, Japan became embroiled in a war with China that eventually merged into World War II. The US sided with China, and began openly and secretly supporting the Chinese, and started cutting off trade with Japan. This increased tensions between the US and Japan. After Germany conquered France in 1940, Japan began taking control of territory in southern Asia that France had ruled, and forged an alliance with Germany and Italy- Japan's leaders knew that they would eventually have to fight the USA. After Japan took full control of French Indochina (modern Vietnam, Cambodia and Laos), the US eventually took the dramatic step of cutting off oil sales to Japan. Meaning Japan would be forced to conquer areas where there was oil, like modern Indonesia.

So in 1941 they decided they would end negotiations with the USA and start the war. They were not going to waste the opportunity to knock the American navy out of the war before it truly began. So they intended to break ties with the US, issue a declaration of war, and attack Pearl Harbor before the Navy could move, all in rapid succession. However, in one of history's great quirks, the declaration of war was delayed and given to the US late, meaning Japan had attacked at Pearl Harbor before telling the US they were at war- which the US saw as a treacherous sneak attack (however, there are theories that say that the US knew the attack was coming but let it happen so they had a reason to join the war).

Before Pearl Harbor, the US and Britain had the largest navies in the world; Japan's was next in line. As an island nation, Japan naturally needed to be able to operate their navy without interference from Allied navies if they wanted to continue conquering territory- airplanes weren't yet able to easily move around all the materiel needed to fight a major war. So by attackign the US Navy, Japan tried to eliminate that threat to their own navy, and hoped to deal a decisive blow that would cripple American morale, possibly even forcing the US to avoid joining the war. Instead it had the opposite effect.