Sentence:  But where the core of NATO <WSD>is</WSD> concerned , the Secretary of State has not only reiterated the United States ' profound attachment to the alliance , `` cornerstone '' of its foreign policy , but has announced that five nuclear submarines will eventually be at NATO 's disposal in European waters . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)