Sentence:  Nevertheless , the consequence of the policy proposed is everywhere subtly qualified : it is `` a possible result , however improbable '' ; `` the worst , and least probable '' result ; `` if it did n't prevail mankind would still be given the opportunity of prevailing '' ; for `` surely anything is better than a policy which <WSD>allows</WSD> for the possibility of nuclear war '' . 
Meaning of the word marked with <WSD> tags:  make a possibility or provide opportunity for; permit to be attainable or cause to remain