If Americans would stop doing drugs, there would be a decrease in the number of people wanting to cross the border as where they came from would be a lot more livable.
The Great Wall of China didn't stop all invasions of undesirables. Genghis Khan (1162 - 1227), the founder of the Mongol Empire, was the only one who breached the Great Wall of China in its 2,700-year-history. In his lifetime, Genghis Khan led his Mongolian army to break through the Great Wall not only once, but several times at Wusha Fortress, Juyongguan, Zijingguan, and Tongguan, etc. These successes were a big help in overthrowing of the Jin Dynasty (1115 - 1234 AD) and founding of the Yuan Dynasty (1271 - 1368 AD).