Imperial Japan was very much a martial society, one in which the armed forces enjoyed a high level of prestige and influence. Combined with an intense nationalism, the militarization of Japanese life had turned the country into an aggressive power, willing and able to wage war. All of the available evidence suggests that the Japanese had been preparing for war in South East Asia since at least 1936. Yet it took the outbreak of World War II in Europe for the Japanese high command to put its long-standing military strategy into effect.

Large parts of South East Asia belonged to European imperial powers, mainly Great Britain and France. Emboldened by its successes in Manchuria, the Japanese invaded British colonial territories, such as Hong Kong and Singapore. The British were unable to put up much resistance to the Japanese, as they were bogged down in fighting the Germans in Europe and so were dangerously overstretched in relation to defending their empire. The Japanese were also able to take advantage of the weakness of a defeated France to invade and occupy French Indochina.

There were reasons why Japan did so well in the early part of World War II in Asia and the Pacific. One reason was because of the devastation our military experienced at Pearl Harbor. Much of our navy and air force was destroyed in the attack at Pearl Harbor. As a result, Japan was able to move freely through the Pacific and Asia with little resistance from the United States. We were in the process of building more ships and planes, and while we were doing that, Japan was able to conquer much of the central and western Pacific as well as parts of Asia. Another reason was that Britain was tied up in Europe fighting the Germans. As a result, there wasn’t much the British could do to help us in the Pacific or in Asia. Once our military was restored in the Pacific, Japan’s advances were stopped. After the battles at Midway Island and Guadalcanal, Japan was retreating for the rest of the war.

Japan had successes early on because it was not fighting anyone who was particularly well prepared and/or armed.  Japan's armed forces were well-trained and had good plans.  They were were ready to fight. By contrast, the forces they fought were nowhere near as ready.  In the Philippines, for example, they faced a few Americans without much in the way of air power and a large number of poorly trained and equipped Filipino soldiers.  Britain's main forces were busy in Europe and their supposedly impregnable fortress in Singapore was not nearly as strong as it was believed to be.

Overall, then, Japan won early victories because it had plenty of well-trained soldiers who were ready to fight while its enemies did not.