I think we average about 20 a year, and a significant number of those are innocent things where ... police or military from Mexico may step across the border because they're not aware of exactly where the line is," Chertoff said.
Build a wall or secure fence. Then there will be no more lame excuses if the foreigners cross it again.""
There is a "LINE". It is called the Rio Grande River, for the most part of the border.
Zapata and Pancho Villa knew exactly where it was, and they crossed it just as deliberately.