The free world, at least dimly aware of these facts, has naturally embarked on a large programme of warning and defence systems. That programme will be accelerated and extended. But let no one think that the expenditure of vast sums for weapons and systems of defence can guarantee absolute safety for the cities and citizens of any nation. The awful arithmetic of the atomic bomb does not permit of any such easy solution. Even against the most powerful defence, an aggressor in possession of the effective minimum number of atomic bombs for a surprise attack could probably place a sufficient number of his bombs on the chosen targets to cause hideous damage.