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