Sentence:  One effect of the spirited give-and-take of these discussions was to focus attention on practical applications and the necessity of being armed with the facts : knowledge of the destructive force of even the tiniest `` tactical '' atomic weapon would have a bearing on judgments as to the advisability of its use - to <WSD>defend</WSD> Berlin , for example ; the pervasive influence of ideology on our political judgments needs to be recognized and taken into due account ; it is necessary to perceive the extent of foreign aid demanded by the Christian imperative . 
Meaning of the word marked with <WSD> tags:  be on the defensive; act against an attack