In the aftermath of the New Zealand mosque attacks, public debate is mounting in Western democracies over the severity of the threat posed by far-right extremists.

The United States and Russia are at odds on so many issues that the upcoming summit could convince their two presidents to give up on better relations and accept estrangement.

Far-right extremists are attempting to incite an insurrection to hasten the downfall of what they see as a deeply corrupt U.S. government. Some could resort to deadly acts of terrorism.

The U.S. counterterrorism response to the September 11, 2001, attacks yielded some remarkable successes and disastrous failures in hunting al-Qaeda. The top terrorist threat today, though, is domesti…