The continental US borders both Mexico and Canada. However, it has many countries close to its borders, less than 100 kilometers (62 miles) away from American soil, such as Russia across the Bering Strait, or the Bahamas and Cuba, close to Florida in the Caribbean Sea. Also, if you count Puerto Rico as part of America, some more countries enter this classification, such as the Dominican Republic or Haiti.
Canada and Mexico both border the United States of America.
Canada and Mexico both border the US. The Caribbean countries as well as the Central American ones are also fairly close.
The two countries closest to the US are Canada and Mexico, which share common borders with the US.
The United States is between Mexico and Canada
Mexico
Canada of course: it has territories close to the Arctic circle.
Tierra del Fuego, shared by Argentina and Chile and close to Antractica.
Canada
Geographically speaking, Canada, Mexico, Cuba, and Russia. Alliance wise, Australia, Mexico, UK, and Canada.
Greenland.
Absolutely nothing, it is as close to perfection as a country can get.
The U.S. or Russia