Walls, fences, and other artificial barriers along national borders have been proliferating in response to a variety of perceived threats to sovereign states such as terrorism and drug trafficking. But by far the most significant rationale today for the construction of new barriers is in response to prospective migrants, ranging from undocumented individuals to massive refugee flows. The European refugee crisis, which reached a peak in 2015, and the repeated arrivals of Central American refugees at the United States border with Mexico prompted, respectively, new barriers in Europe and a determination by former President Trump to extend existing barriers in the United States along its border with Mexico. Often such initiatives have more to do with fear-mongering and other political motives than with effective control of migrants given the characteristic reliance of migrants on normal avenues of approach and ports of entry.