Militarily speaking, Japan had a great deal of success early in WWII. Of course, by the end of the war, all of that success had disappeared and Japan was completely defeated.
Japan had essentially everything its own way until mid-to-late 1942. Up to that point, Japan had advanced on the Asian continent and in the Pacific. It had control of much of China and Indochina. It took Singapore and Hong Kong and the Philippines and Burma. It took what is now Indonesia and controlled much of New Guinea. By June of 1942, Japan was at its peak.
From there, Japan stopped advancing. Defeats at Midway and later at Guadalcanal were the beginning. From then on, US forces moved closer and closer to Japan, eventually coming as close as Okinawa before the atomic bombs ended the war.
Japan had tremendous success prior to the Americans entering the war. They had put almost all of eastern Asia under their control with some of the most brutal attacks on civilians in world history...essentially terrorizing China into submission. (See the Rape of Nanjing)
One of the enduring arguments about World War II involves the Japanese decision to involve the Americans. There is a reasonable argument that with American pre-occupation with Hitler, Japan could have pretty easily solidified their grip on Asia until the inevitable defeat of Germany. Many within the Japanese inner circle argued for such a strategy. The thinking was that the United States would have no interest in fighting a second war and would tolerate a Japanese empire in Asia. We will never know, of course, if that is true. What is not subject to debate, however, is the rapid demise of the Japanese once they miscalculated the US response to the attack on Pearl Harbor.