while it's counterproductive and stupid to throw rocks at an ambassador, just because he's there by your own agreement and invitation, it is many times worse for an ambassador to get mingled in the internal conflicts of the host country and visit an opposition leader of the very government hosting you. this should be an automatic persona non grata status. it's only designed to be deliberately provocative. US could recall the envoy and close the mission if it opposes the syrian government, but it cannot have an ambassador publicly court an opposition leader who's trying to overthrow the government. i wonder if US is trying to create an incident as a prelude to an attack.
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from the article: U.S.: Attack on envoy in Syria part of ongoing intimidation campaign