Question

Why did the fighting in the Pacific result in so many casualties for the United States​

Answer 1

Japanese tradition of death before dishonor meant that there was no surrender for Japanese soldiers, and hence a massive amount of American and Japanese lives were lost, due to Japan clinging to every island they had in the Pacific. On a larger scale this never surrender mentality made the Americans come to the conclusion that they needed a complete and total surrender to be forced on Japan, but also because of the casualties that would likely be taken from a full scale invasion of the Japanese Home Islands, would lead to the idea of simply dropping two Atomic Bombs on some of Japan's most prosperous cities.