Leo was strongly opposed to the use of atomic weapons on any sort of population, but especially against non-military targets. The United States' view before World War II of being very opposed to bombings of civilians was one of his main reasons for working on the project there. He hoped Japan and/or Germany would surrender just from a mere demonstration of an atomic bomb, and wrote the Szilard petition, which asked for there to only be a demonstration and not an attack. The end of the European campaign and high casualties in the Pacific campaign made President Harry Truman decide to follow his aides' recommendation to use atomic weapons on Japanese cities in an effort to destroy their resolve.
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Hiroshima after the bomb.