Every year, on December 7th, we note the anniversary of the Japanese attack on Pearl Harbor, and rightly so. This event catapulted the United States into World War II and brought it permanently onto the world stage. Rarely, though, do we think to ask why it happened: why did the Japanese decide to launch a pre-emptive attack against a hostile but ultimately unthreatening nation? The answer can be summed up in one word: oil.
The Imperial Japanese military machine ran on oil (unlike the civilian economy, which was mostly coal-powered at the time). However, most of Japan's oil came from two sources, both of them controlled by other nations: the Dutch East Indies (now Indonesia) and the United States of America. To Japan's leaders, having such an important resource under the control of hostile foreign governments was intolerable. They thought that the solution was obvious: go out and conquer the oil fields.
Specifically, they intended to conquer the Dutch East Indies. However, to secure the route, they felt it necessary to conquer everything in between, including the Philippines. The Philippines being a US possession at the time, this meant that conquering the Dutch East Indies would involve fighting a war with the United States.
Japan's leaders were not terribly concerned by the prospect. They figured that after they destroyed the US Pacific Fleet at Pearl Harbor in a surprise attack and conquered the Philippines, the defeated Americans would meekly accept Japanese domination of the Pacific. They didn't think that a defeated enemy would keep on fighting them. Besides, they really wanted that oil, and their plans required their conquered enemies to accept defeat. So, not only did they not prepare for a long war, they refused to even consider the possibility of having to fight one.
To the dismay of Japan's leaders, their defeated enemies kept on fighting them. The territories that had been so swiftly and easily conquered refused to stay conquered, and bit by bit, Japan's newly won empire was lost. In the end, so was the war.
The lessons for latter-day empire-builders should be obvious: first, conquest may be easy, but control is hard; second, you should always prepare for the worst; and third, refusing to face reality is a certain guarantee of ultimate failure.