Final answer:
The United States is bordered by Canada to the north and Mexico to the south, with the U.S.-Canada border being the longer of the two.
Step-by-step explanation:
The United States shares its borders with two countries: to the north with Canada and to the south with Mexico. The U.S.-Canada border is significantly longer than the U.S.-Mexico border, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west. The border with Mexico is characterized by arid and open regions, including the Chihuahuan and Sonoran deserts, which present challenges for managing immigration and security.