You're asserting that US interference in the Middle-East has caused present Sunni-Shia conflict, which may well be correct. But I think your supporting point of "it doesn't happen in other parts of the world" is weak.

Of course they aren't tracking down and killing each other in the US, because they're both minority groups and probably rarely even run into each other.

That doesn't say anything about whether they would be doing so if they were both in the majority and their sectarian conflicts were the dominant aspect in society.

    > The difference in my eyes is that Muslims in the Americas have
    > governments they can depend on for arbitration.

So if the governments in the Americas collapsed do you think the Sunnis and Shias there would go right back to systematically killing each other? Could you e.g. say the same thing about the Amish and the Mormons? Or is there a pre-existing divede between these people that really has nothing to do with external factors?

You can't both say that Sunni & Shia conflict happens in certain areas of the world because of US foreign policy, and also suggest that Sunni & Shia conflicts would arise between Muslims living in the US who aren't subject to US foreign policy.