Sentence:  Nevertheless , the consequence of the policy proposed is everywhere subtly qualified : it <WSD>is</WSD> `` 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 allows for the possibility of nuclear war '' . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)