This will happen, one catalyst can be a global economic depression which seems likely in the near future, which in turn can be triggered by major acts of terrorism laying our fragile monetary system bare.
On the side of America's enemies are the Democrats, who, among other things destructive of America, encourage third world immigration because the immigrants vote overwhelmingly Democrat and keep these people in office. If this trend continues, America will be overwhelmed by these immigrants and the United States will likely fall.
On the other hand, the September 11 Massacre galvanized the American spirit and united Americans as they had not been in decades. It is possible that Americans under seige will unite in a common defense and that this will spur the American economy to unprecedented growth, as World War II did. And furthermore, it may bring the immigrant population into the mainstream, united with the rest of America in the common defense.