The intensifying scrutiny of the role of “great replacement theory” in helping to inspire the horrific mass shooting in Buffalo is a reminder to the country: We could have seen it all coming.In retrospect, the 2017 white supremacist and neo-Nazi rally in Virginia now looks like a harbinger of much of what has followed. That “Unite the Right” gathering was inspired by the conspiracy theory that elites are scheming to extinguish Whites in Western countries by replacing them with imported non-White immigrants, as evidenced in chants of “Jews will not replace us.” Alleged mass shooter Payton Gendron’s online screed trafficked...