What if Britain had supported Mexico against the United States?

What would have happened if Britain had intervened on Mexico's side in the Mexican-American War?

The United States would have faced a dangerous two-front war, with the Royal Navy overwhelmingly stronger at sea, but Britain had too little interest in Mexico to intervene.

British intervention on Mexico's side could have supplied Mexico with substantial weapons and material. The Royal Navy would also have posed a severe threat to the much smaller United States Navy. The resulting counterfactual includes a two-front war for the United States. The explanation for British restraint rests on Britain's limited national interests in Mexico, with a later distinction between private interests and government policy.