Countries That Have Ever Bordered Mexico

For the Rio Grande I decided that the failure of the rebellion as well as the lack of any meaningful international recognition excluded the state from being a country proper.

Chiapas seems to have been, on paper, a part of the Mexican Empire, however like most of Mexico at that time, it saw little attention from the federal government, instead being run mostly by the local elites. The area did declare its own independence from Spain separate of Mexico but as far as I can tell Mexico obtained sovereignty over the area, reasserting this claim once the Federal Republic of Central America became independent of Mexico.

As far as I could find Soconusco similarly made a claim of sovereignty on its own but never seems to have pursued any independence from Mexican or Central American states, lying in political limbo while regional powers disputed the area.

All that said I am surely no expert on Mexican history and would gladly be wrong.