It was the only option they had at the time. The US, UK, and Dutch in 1940 embargoed all trade with Japan this includes copper, iron, steel, rubber, and most importantly oil. Japan relied on oil imports from the US to keep its war machine going. The US did this in protest of Japan’s aggressive actions in Asia. Japan now has two choices: to give up all gains made in China and Asia, or go ahead with their Southern doctrine. Nanshin-Ron was the plan of seizing Southeast Asian and European colonies for themselves. This includes the Dutch East Indies which produced enough oil to keep Japan going. And British Malaya, which it and the Dutch East Indies held most of the world’s rubber supply. These regions also had a lot of other natural resources which would help with their lack of imports. With Europe in chaos it would be easy pickings for Japan who’s navy could stop any reinforcements coming from Europe. Plus many of the troops in these areas would be ill equipped and not as well trained as the veteran Japanese soldiers.

However one problem stood in their way: the US fleet. Japan knew that this was the only thing that could stop them and they had to knock out the Pacific fleet. With it out of the way the rest of the Pacific would be a cakewalk. While the attack on Pearl Harbor was a devastating blow to the US, it didn’t do as much damage as you might think. The Carriers were not even found and were all undamaged, the subs also in Pearl Harbor were untouched, the repair yards were also lightly damaged, but most importantly the oil storage yards for the whole Pacific fleet were untouched. If these tens of millions of gallons of oil were destroyed, the Pacific Fleet would not be able to commit operations for sometime.

While attacking the US fleet at Pearl Harbor was an audacious plan, it was Japan’s only choice. As Japan was not about give up its gains in China and it was running dangerously low on everything after the embargo.