From the Telegraph obituary:
When, in 1950, the implications of hydrogen bomb production were being aired, Bethe expressed his hope that America would never use the bomb first. On television he warned that H-bomb clouds could annihilate life on earth, and said that he would not like to see America's strategic plan centred on the weapon.
Decades later, in 1997, when there was much debate in America about "pure fusion" nuclear weapons, Bethe wrote to President Clinton calling for an explicit ban on "all physical experiments, no matter how small their yield, whose primary purpose is to design new types of nuclear weapons".
Believing that "success is unlikely" with pure fusion weapons (conventional nuclear weapons need a fission trigger to induce fusion), Bethe was concerned that American scientists might none the less solve the problem, and that they would then be unable to keep their solution to themselves. "There are always temptations in the laboratory," Bethe explained. "I want our country to go on the record that we really do not want further weapons developed. "