What was Kennedy referring to when he spoke of the deadly atom

Terrorismthe threat of nuclear annihilationa dangerous unit of matter

Here is where he said it.

"""But neither can two great and powerful groups of nations take comfort from our present course - both sides overburdened by the cost of modern weapons, both rightly alarmed by the steady spread of the deadly atom, yet both racing to alter that uncertain balance of terror that stays the hand of mankind’s final war."""