Sentence:  To the extent , then , that declining U. S. prestige means that other nations will be tempted to place their bets on an ultimate American defeat , and will thus <WSD>be</WSD> more vulnerable to Soviet intimidation , there is reason for concern . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)