North - Canada / South - Mexico .
Assuming this is a clue for a US-based crossword puzzle, then the answer is Mexico.
if you mean the countries then mainly Guatemala but also Belize & even the usa in a few odd spots i kid you not
The United States neighbors Mexico to the North; Guatemala and Belize neighbor Mexico to the South
Guatemala is. See related questions.
Colorado is bordered by New Mexico to the south. Denver is the capital city in Colorado.
Mexico is the southern neighbor of the U.S.
The closest southern neighbor would be Mexico.
Mexico is the immediate neighbor to the United States, just south of that country. As Paraguay is a South American country, to visit the United States you would have to pass through or fly over Mexico. Therefore, Paraguay is closer to Mexico than to the US.
Old Mexico
The nation that is Guatemala's neighbor to the north is Belize. Mexico is another country that is north of Guatemala.
If the neighbor's fence is on your property in New Mexico, you can ask them to remove it. If they refuse, you can take them to civil court.