mexico
Tennessee borders 8 states (as does Missouri), the most of any.
If you are referring to the southern region of the United States, then Mexico borders the southwestern US.
The southwest region of the United States borders Mexico.
Colorado borders the mountain region.
the answer is Mexico
The costal plains
First of all, South America is a continent, or at least a region of the Americas. It is not a country. Second, of the two, Canada borders the US.
The region that borders the Southwest region on the east is the South region of the United States. This area includes states such as Texas, Louisiana, Arkansas, and Oklahoma, which share borders with the Southwest states like Arizona and New Mexico. The geographical and cultural characteristics of the South influence the dynamics of the Southwest region.
The Sahel borders the Sahara.
tidewater
Ladakh