Former President George W. Bush issued a dire warning on Saturday while commemorating the 20th anniversary of the September 11, 2001, attacks, saying international extremists are not the only threats to the country's safety. In the wake of stark political and social division in the U.S., Mr. Bush said that "violence that gathers within" is equally as dangerous as extremists abroad. "We have seen growing evidence that the dangers to our country can come not only across borders, but in violence that gathers within," Mr. Bush, who was president on 9/11, said. "There is little cultural overlap between violent extremists...