What new weapon made allied leaders worry about the cost of invading the japanese mainland?

The correct answer is the atomic bomb.

Shortly after the Allied powers had success in defeating Germany in the European theatre, this group turned their attention to how to defeat Japan in the Pacific theatre. In this case, the Allied powers (specifically the United States) considered a land invasion first. This would be similar to the land invasion conducted in Normandy, France. This type of invasion was predicted to result in millions of casualties.

However, instead of having millions of casualties from both the Allied and Axis powers, the US government considered using the atomic bomb. This bomb would result in less casualties and a quicker close to World War II. This weapon completely changed the way the Allied powers thought about their strategy against Japan.