Sentence:  The concept of the Middle Kingdom at peace , strong and united under a forceful ruler , which had <WSD>been</WSD> only a longed-for ideal in the time of the Warring States , was finally realized by the establishment of a Chinese Empire under the Ch ' in dynasty ( 221 - 207 B. C . ) . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)