Several European countries have erected fences to keep migrants out, and, according to the numbers, every case appears to have a large impact. Hungary was a popular pathway for refugees on their way to Germany during the fall. When the daily illegal border crossings were at 7,000 per day, Prime Minister Viktor Orban decided to erect a fence along the border to Serbia and Croatia. The result speaks for itself: When the fence went up Oct. 17, the influx went down to 870 from 6,353 only a day earlier. Illegal border crossing were steadily below 40 per day throughout the...