What did Japan accomplish militarily during World War 2?
Japan accomplished the same thing that it's partner Germany did; they both accumulated a lot of foreign territory (countries). On the technical side, the Japanese advanced the uses of the aircraft carriers, which included outstanding fighters, dive bombers, & torpedo aircraft; massive sized submarines, the largest in the world at that time; aircraft carrying submarines; Germany, Japan & Italy all used midget (mini) submarines; the most powerful battleships in the world - Yamato & Mushashi (Nine 18-inch guns/rifles, 70,000 ton displacement), and the most powerful torpedo during WWII, the "Long Lance."
US occupation did NOT affect Japan DURING the war, but after it, since the war ended with the occupation of Japan. So, the occupation denied Japan of many soldiers, military facilities etc, rendering it quite useless militarily. Afterwards, the US made immigration to and from Japan easier, enriching both countries. That is what happened in most sectors, technology, science, industry, the American 'way' was introduced, while Japan developed on these principles.
After Japan emerged from its isolationist or Sakoku period, it began its Industrial Revolution. This period was known as Meiji, and it changed Japan's political, cultural and economic structure to welcome the modern world. It colonized Manchuria, Korea and Taiwan during this period, and it's growth skyrocketed until it was able to compete with other world powers both militarily as well as economically.