We sometimes ascribe "fierceness" to soldiers, such as ISIS fighters today, when what we really mean is that they refuse to surrender when all is lost. I would argue that the United States Marines...the victors...were the fiercest warriors in the Pacific. The Japanese were merely idiotically stubborn.
And dead.
The Japanese soldier was very well trained, and very willing to kill the enemy. Studies showed that US soldiers were very hesitant to kill. Only about 12% of US soldiers would shoot to kill (unless they were in imminent danger), because they were hesitant to kill and very much preferred not to.
Conversely, over 90% of all Japanese soldiers would shoot to kill every time. So the US studied the Japanese kill program to learn how they did it, and we modeled our own after their kill program...we just made a more civilized version.
But I will agree US Marines were very fierce, and our resolve was every bit as much as was theirs. I think that really surprised the Japanese leadership, because they had never ran into an enemy with the resolve of the US Marines...Nor had they ever seen soldiers who were so incredibly competent.
But that competence came at a price. We very quickly learned, that to fight the Japanese, you need more than just the 3-1 attackers advantage.