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.
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 tasted great success in the early days of war in the regions of Asia and the Pacific. Within a few months of the Pearl Harbor Attack, Japanese forces controlled a large portion of the Pacific including, Guam, Wake Island, Hong Kong, Singapore, Dutch East Indies, Burma and the Philippines. There are several reasons for this initial success. Most of the conquered countries were small in size and were not prepared for war. Japanese soldiers were trained for combat and warfare and hence had a huge advantage over these countries. The war was led by navy and fighter planes, both of which were new to these territories and provided huge advantage to japanese soldiers. The local forces were mostly untrained and offered hardly any resistance. The European forces were busy with the fighting in Europe. Hence lack of any retaliatory measures ensured Japan's initial success.