Question

in 1945, what concern did the allies have about invading the japanese mainland with ground forces?
they were worried they did not have enough soldiers.
they were worried there would be too great a loss of life.
they were worried the japanese were developing an atomic bomb.
they were worried the japanese had a stronger fleet in mainland ports.

Explanation:

The planned invasion of Japan (Operation Downfall) was expected to result in extremely high casualties for both Allied and Japanese forces due to the fierce resistance the Japanese were likely to put up on their home - soil. This high potential for loss of life was a major concern for the Allies.

Answer:

They were worried there would be too great a loss of life.