Sentence:  Here then is our problem : aircraft are vital to winning a war today because they can perform those missions which a missile is totally incapable of performing ; but the airfield , on which the aircraft <WSD>is</WSD> completely dependent , is doomed by the missile . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)