Sentence:  This is true for two reasons : because Communism <WSD>is</WSD> both doctrinally , and in practice , antithetical to these conditions ; and because Communists have the will and , as long as Soviet power remains intact , the capacity to prevent their realization . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)