Answer:
In the statements prior to the given statement, the author makes their opinion clear. The introductory sentence states, "How can a human being with any claim to a sense of moral responsibility deliberately let loose an instrument of destruction." Off of this statement alone, it is evident that the author strongly questions the morality and ethicalness of dropping a nuclear bomb. It goes on to state "This is not war: . . . this is pure nihilism."
Thus when the author states "strikes at the very basis of moral existence?" they strongly believe that dropping a nuclear bomb is immoral and unethical, they question the meaning of being human if "the very foundations or morality are overthrown" by dropping a nuclear bomb.
I hope this helps.