actually you can't prove that, it is a logical fallacy, and definitely not supported by the facts.
The tension between sunni and shite's has been building for centuries. Even without the US military and stupid foreign policies those two groups have been building to a islamic civil for a few hundred years.
Just like christianity did when it turned 1400 there will be purges, burning, and random fighting between the various islamic sects. The very reason the USA has freedom of religion was to prevent any one christian religious sect from controlling the government at the expense of others.
Muslims are doing the exact same thing. only with automatic weapons, and RPG's.
Those terrorists would likely have still been there.