Canada!
North America and South America share a land border with the United States.
45 dagrees north
Canada in the north, and Mexico in the south.
The Atlantic Ocean (more specifically the North Atlantic), which forms the eastern coast of the US.
Rio Grande forms some of the border.
The Rio Grande forms part of the southern border of the US, separating the United States from Mexico. It is the fourth-longest river system in North America.
Canada is on the north in relation to the US. The southern part of Canada forms the border with the northern part of United States.
The only two countries with land borders with the US are Canada to the north and Mexico to the south.
The Rio Grande forms part of the US border with Mexico, while the St. Lawrence River forms part of the border between the US and Canada.
They are not contiguous. Alaska is a part of North America, but has no border with any US State. It does have a maritime border with Russia and a land border with Canada. Hawaii is not in North America, but is a US State located in the Pacific Ocean.
Canada forms the most northerly border of the U.S.