The experience of the General (later Field Marshal and Viscount) Slim was that the reason behind Japanese successes was the same as the reason for them taking high casualties. The reasons are twofold - the behaviour of the commander and the individual soldier.
Individual Soldier -
Individual Japanese soldiers rarely surrendered. They would routinely fight to the death, even in obviously hopeless situations. This is an advantage, in that a defence can be made stronger if the defenders will not surrender despite casualties and an attack is harder to blunt if the individuals will continue to attack regardless. It carries with it the obvious disadvantage that unless the commander orders a withdrawal, defending units will be completely lost and an attack that fails will fail with very heavy losses on the Japanese side. Slim noted that about ten soldiers was the most that could be expected to be captured from the destruction of a roughly battalion sized formation (I don’t have the book handy, so will have to check numbers later).
Commander -
In battle, it is important to have a reserve. That is, troops not engaged by the enemy who can be deployed to exploit successes or block enemy counter attacks etc. Having your own reserve committed while the enemy commander retains his is often a ticket to defeat as he has freedom of action while all your units are engaged. For this reason, commanders are reluctant ever to commit all their reserve to battle.
Early Japanese successes in Russia are partly attributed to the willingness of the Japanese Commander to commit his entire reserve to battle. This had the effect of tipping the balance of a closely fought battles in favour of the Japanese. This was repeated across China and through Burma.
Soldier and Commander Combination -
Because the individual soldier would not surrender, and the commander would almost routinely commit his reserve, the Imperial Japanese Army could mount very effective attacks and resist enemy attacks better than might be expected by a similar formation from the other major combatants. This strength was the cause of the major vulnerability. If an IJA attack could be stopped, the casualties taken by the Japanese would be high and the commander would have no reserve, making him vulnerable to a rapid counter attack. Similarly, defeating a Japanese defensive position was tough, but succeeding would then leave an greater opportunity to exploit the success than would be expected against, say, the Germans.
Thus, although the characteristics described above were a vital part of the success of the IJA they were also the source of a key weakness, one that was exploited by Slim in Burma with great success.
