Ok, totally not the point ... but a big chunk of what is now the US was Mexico, and the Treaty of Guadelupe Hidalgo (and to a lesser extent the Gadsden Purchase

Ok, totally not the point ... but a big chunk of what is now the US was Mexico, and the Treaty of Guadelupe Hidalgo (and to a lesser extent the Gadsden Purchase) officially handed it over -- so I think Mexico may actually have formally agreed that we're not North Mexico.