People (Nations/Countries) take the paths offered by their particular geography. E.g. people that live in forests will use trees to make a living; people that live near the ocean will live off of the sea (fishing).
Russia intermingled with Europe as they were the closest.
India intermingled with China as they were the closes; and Britain as they had a century of ties with them.
Mexico has had a long relationship with the US. A close relationship: California, Utah, Nevada, Arizona, and New Mexico were once part of Mexico.