They have been used in warfare twice, shortly after they were created. For this reason, many Americans associate the use of nuclear weapons with ending a particularly horrible war. If the Japanese had used these weapons or the Germans, our view of them as Americans would undoubtedly be different. But for most Americans the association of nuclear weapons and ending World War II is indelible and positive. In the past 50 years, though, we've had an opportunity to shed further light upon the weapons themselves.
Nuclear weapons are instruments of mass slaughter.
They can destroy whole cities. Used in a coordinated attack, they can destroy civilization and much of life on Earth. They can break the link between the past and the future by cutting off the future. Nuclear arsenals have forced us to consider for the first time in human history the possibility of not having a future. Thus, regardless of whether the use of nuclear weapons at Hiroshima and Nagasaki was justified, the risks of using nuclear weapons in today's world could not be justified.