Despite what Donald Trump and many other politicians have told you, the major threat to America isn’t Muslim extremism. In fact, statistics show that the real danger lies with domestic extremists who aren’t of the Muslim faith. The New York Times reported back in June that since Sept. 11, 2001, almost twice as many people have died at the hands of white supremacists and other non-Muslim extremists than by radical Muslims. Using data compiled by New America, a Washington Research center, a study found that 48 people have been killed by extremists who are not Muslim—including the mass killings in...