Sentence:  ( Since the time-span of the nation-state coincides roughly with the separate existence of the United States as an independent entity , it <WSD>is</WSD> perhaps natural for Americans to think of the nation as representative of the highest form of order , something permanent and unchanging . ) 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)