If they lie to us infidels, and begin to decapitate people, they are breaking U.S. law. If caught, they will be charged with murder in the U.S. court system and if convicted, they will face the penalty. A religious court can not legally judge and give out a death sentence. It is against U.S. law, and Sharia courts have no authority in those areas in the U.S. Just as Bet Dins can not legally judge and give out the death sentence.
They dont see themselves as ultimately under US law. They have no respect for our laws. And they are not going to let US law get in their way. They may not do public beheadings; at least, not at first. What's going to happen is we are going to wake up and hear about someone getting kidnapped and later find their decapitated body or something.
There are 30 murders a day in the US. To start laying blame in the Muslims is a bit naive. We are our own worst enemy by far