If some terrorist camp over there thinks that the building of that mosque has some historical significance as a marker for their success, it makes no difference. That is, they aren't terrorists because that gets built. And, that they take some perverse delight in it is really of no consequence other than that it bothers some people here -- even though it makes no difference in practical reality.
On the other hand, if the mosque is viewed as a place or a cause that forces us to confront the lack of mutual understanding and respect then there is at least a chance that somewhere down the line it, and places like it, will allow generations of both Muslims and American to decide that their antagonism is something ginned up by people who want to take advantage of it.
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i'd say a mosque rising out of the world trade center is the type of symbol that could rally terrorists for generations. you seriously underestimate that symbolism.
please explain why a mosque built there would ever help mutual understanding and respect?