At present the best defense against the atomic bomb is to not be there when it goes off.

attributed to Groves by Vice Admiral William H.P. Blandy, in Blandy, "Operation Crossroads: The Story of the Air and Underwater Tests of the Atomic Bomb at Bikini," Army Ordnance 31, no. 160 (January-February 1947), 341-343, full quote as listed above is the pull-quote on page 341, with attribution on page 343 ("At present, the best defense is distance, or as General Groves put it, the best defense is to not be there when it goes off").