Countries with boarder walls:
Israel-West Bank:
The border wall — actually much more a security fence — was constructed in the wake of the 2001-2002 terror campaign in Israel. Almost immediately, the number of successful terror attacks in the Jewish state dropped by
90 percent. The 2013 upgrade reduced illegal incursions at the border by an average of
99.4 percent.
Hungary:
In 2015, more than a million migrants poured into Europe from North Africa and the Middle East. That year, 410,000 crossed the Hungarian border. On a per capita basis, that would be like 14,000,000 illegal aliens entering the United States in a single year. Completing the fence cut the number of daily crossings essentially to zero.
Germany (Cold War Era):
The Berlin War. Not a good thing but quite effective. Defection was a serious "problem" for the East Germans. Between 1945 and 1961, over 3.5 million East Germans walked across the unguarded border. As the graph below demonstrates, the wall cut defections more than 90 percent.
The wall was even more effective than this graph make it seem, because these figures include
legal emigration. As the next graph shows, most of the movement to the West was legal. The wall and the “rampart” slashed defections to just 185.