When the Japanese fought so strongly to hold on to the islands what did the American military leaders begin to fear?

In he Japanese army soldiers fought to the 'last man'. and the they even started a programme of kamikaze flight, which were suicide missions. The aim was to eliminate as many Allied targets as they could. During the famous tactic of island hopping, on a small island 23,000 Japanese fought to the last man and 20,000 Americans died (capturing about 8 square miles altogether). So, the Americans saw with fear that the Japanese did not weaken in determination to fight. They could have about 5 million soldiers, and the Americans thought that they would not be able to destroy such an army. If they tried, millions of soldiers were to die.