If you are writing for a British publication, use “defence,” but the American “defense” has the advantages of greater antiquity, similarity to the words from which it was derived, and consistency with words like “defensible.” The pronunciation used in sports which accents the first syllable (“DEE-fense”) should not be used when discussing military, legal, or other sorts of defense.
People in sports use “defense” as a verb meaning “defend against,” as in “the team couldn’t defense that strategy.” Outside of sports talk, “defense” is never a verb.
See also offence/offense.
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