The Collaborative International Dictionary of English v.0.48:

de-escalate \de-escalate\ v. t.
   to reduce in intensity (a crisis or a war).
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

de-escalate
    v 1: diminish in size, scope, or intensity; "The war of words
         between them de-escalated with time"
    2: reduce the level or intensity or size or scope of; "de-
       escalate a crisis" [syn: de-escalate, weaken, step
       down] [ant: escalate, intensify, step up]