South African Republic (SAR) is Botswana's southern neighbor. http://www.mapsnworld.com/world-map-political.html
Mexico is the southern neighbor of the United States.
USA is the closest southern neighbor of Canada.
Mexico is the closest southern neighbor for most of the US.
Egypt.
alabama
Mexico is the southern neighbor of the U.S.
armenia
Mexico is the country that borders the southern United States.
Panjab
Mexico
If the question means USA for America then Mexico is on the southern border of the USA.
Mexico