It is important indeed to tell potential recruits for Al Queida that they will not be welcome in Iraq. If it is tough to get recruits attrition , however slight , will cause desertion.
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Most potential recruits would be Iraqis, so telling them they are unwelcome in their own country would be like telling you you are unwelcome in the US or in Georgia. I think it is safe to say that an Iraqi that enlists in Al Qaeda does so because he feels, that as a Sunni Arab (Shiites are never going to join Al Qaeda) that this is best for both him and Iraq, and is in fact, the Will of Allah.