We Americans are lucky, though we seldom reflect on it, that we have good neighbors. In East Asia, Japan, South Korea, Taiwan, and the Philippines face challenges from China over islands they have long claimed in the East China Sea. In Europe, Germany and other prosperous nations face demands for subsidies from debt-ridden nations to avoid the collapse of the euro. When Southern Europeans look across the Mediterranean, they see Muslim nations facing post–Arab Spring upheaval and disorder. The United States has land borders with just two nations, Canada (on which more on another day) and Mexico, where Barack Obama...