It must help that Mexico’s murderers have decamped to America…

One is that the 21st-century spike has not undone a massive reduction in homicide that Mexico has enjoyed since 1940, comparable to the reductions that Europe and the United States underwent in earlier centuries. The other is that what goes up often comes down. The rate of Mexican homicide has declined in each of the past two years (including an almost 90 percent drop in Juárez from 2010 to 2012)…