Tactics and Strategy

Strategy

Hokushin-ron

The Northern Expansion Doctrine favored especially by the Kodoha faction of the Imperial Japanese Army. This would involve coordinating with Nazi Germany to sieze the eastern parts of the Soviet Union. In real life this doctine was rejected due to the battles of Khalkin Gol in Mongolia, where the Japanese realized their mechanized force was not sufficient to risk taking on the Soviet Union at this phase. Additionally the natural resources needed by Japan were not in as great of supply as they were in southeast asia.

Nanshin-ron

The Southern Expansion Doctrine favored by the Imperial Japanese Navy. This plan was the one implemented in real life, which expanded the war to the European colonies in the pacific like the Dutch East indies, Burma, Malaysia, and the Philippines. These regions are rich in much needed oil and rubber, and Japan's navy is large enough to pull it off handily.

Naval Primacy and Kantai Kessen

Japan is rife with Military Politics, and the Navy typically wins this battle. Modeling their military strategy after the British Empire, another island empire, the Japanese seek to dominate their sphere of the globe with an unstoppable fleet. This means the Navy gets most of the funding, leading to the most modern ships and designs, while the Army is left with old Great War era guns and tanks.
The Navy's main theory was that of "Kantai Kessen" or Decisive Battle. This theory was based on the Japanese success at the Battle of Tsushima in the Russo-Japanese War, where Japan destroyed Russia's navy in a single battle. Rather than focus on attrition or other long term theories, Japan was always fishing for the ability to crush the enemy quickly and at once since they would always lose a long term battle of industrial capacity.

National Spirit

The Japanese military leaders believed that the Western powers were weak and decadent, and if pushed, even my a less powerful nation like Japan, they would fold like paper tigers. The Japanese beleived that their people's will to fight would outlast whatever industrial advantage or paltry will the west had. Peace negotiations would then commence on terms favorable to the Japanese.
Combined with Kantai Kessen, this highlights a critical flaw in Japanese strategy: script writing. The Japanese would believe that things would occur in such a way, and wed themselves to these theories, but reality often did not oblige these delusions. The Americans were never going to be defeated in a decisive battle and they had far more will to fight than the Japanese presumed, yet the Japanese clung to these beliefs till the end.

Tactics

Bayonet and Banzai Charge

The Japanese had a strong spirtual affinity for the blade, be it the sword or bayonet. A comparatively disproportional amount of effort was put into training soliders on the bayonet compared to other nations. This became coupled with mass charges into the enemy lines dubbed "banzai charges". While these charges had legitimate effectiveness against disorganized armies like the Chinese, they were little more than suicide charges against the Americans. In fact many banzai charges were used for just that, a means of dying an honorable death in combat when things had long ago become unwinnable. Surrender was after all, never an option.

Kamikaze

Later in the war Japan had lost the edge by a large margin and resorted to the infamous kamikaze tactics. Most notably were the fighter planes laden with explosives flown straight into ships. More effective than these novice pilots trying to go toe to toe with experienced enemy pilots flying more advanced aircraft, kkamikazes gave Japan what were essentially guided bombs.
Kamikazes were not limited to just pilots however. Kamikaze lunge mines, which are explosive charges attached to poles would be rammed into tanks killing the weilder. Kamikaze submarines, boats, rockets, and scuba-divers were all created and used to varying extents. While not as effective as a skilled pilot of soldier, Japan was in short supply of such people late in the war, so kamikazes helped even the odds a bit.

Defensive Tunnel Networks

In order to fortify their inner perimeter of islands, Japan dug elaborate tunnel netowrks through mountains which prevented the American's superior firepower from destroying the Japanese at range.