Presumably you mean during World War II? Germany was obviously the other possible candidate but for a variety of reasons it couldn't be done (the main one being, they didn't have the atomic bomb ready to use while Germany was still at war). But they did "consider" it.

If you mean after World War II — during the Cold War, the US had standing plans on the books for nuking the Soviet Union, the Warsaw Pact, China, and North Korea. It "considered" using nuclear weapons during the Korean and Vietnam wars (but didn't).