I understand that the dictionary has those equivalents, archijacq, but somehow it doesn't sound right to me when talking about " une douche froide sur l'enthousiasme " . Maybe BE uses it in a different way than AE...
I think the sentence and the meaning should be all negative, first I thought that douché; has a positive meaning because the sentence after that one doesn't seem appropriate when I consider it negative. But then after I read the whole paragraph I think dampen or put a damper will suit the meaning. like;
hearing his fears (about their enemies), has put a damper on their enthusiasm.
the translation is not in English; but if I translate it into English; I would do it like this.