英文關鍵字
abstract
america
american
arizona
background
barbed
barricade
barrier
border
boundary
california
chain
crossing
customs
danger
defense
department
desert
division
enforcement
fence
forbidden
freedom
government
homeland
ice
illegal
immigrant
immigration
international
mexican
mexico
military
national
nature
patrol
protection
security
separation
smuggling
states
surveillance
united
us
usa
wall