Question

for what other reasons would an invasion of the Japanese islands be a concern for American military planners?

Answer 1

It became a concern of the military planners due to the high casualty the marines suffered during the taking of Iwo Jima. Also the battle proved that even though the battle was an American Victory the Japanese army is prepared to die defending and willing to wage a suicide attacks even though they already know that they lost the war. That also attacking the Japanese Home Islands would be a costly attack of lives of american soldiers and the new strategy develop in Iwo Jima would be an arduous battle if the Americans would invade the Japanese Island with the Japanese people that is already to prepare to die fighting.

Answer 2

Japans geography was a concern for American military planners.

There was only a very small portion of their coastline that was suitable for military conditions. Since this was the case, the US only had a very specific spots where they could invade.

Japan knew this and were equipped and prepared to deal with any invasions by the US