MEXICO, the republic bordering the Commonwealth's western provinces of Louisiana and the Californias, with which the Commonwealth fought a brief and, on this contributor's assessment, somewhat reluctantly prosecuted war in 1846 and 1847 over the disputed southern reaches of Louisiana's own purchase boundary. The dispute itself was of the ordinary kind that a frontier drawn by treaty across country neither government had properly surveyed will generally produce: grazing rights and river-crossing claims contested by settlers on both sides for the better part of a decade before either Westminster or the Mexican government troubled to press the question to a formal settlement.
The fighting, once begun, was decided within a single campaigning season, the Commonwealth's standing forces in the Californias and along the lower Louisiana frontier proving considerably better supplied and considerably more numerous than the Mexican garrisons opposing them, and the Treaty of the Rio Bravo (q.v.), signed in 1848, fixed the river of that name as the settled boundary between the two republics from its mouth to the higher country beyond El Paso. The Commonwealth acquired no territory by the settlement beyond confirmation of the boundary its own surveyors had already claimed before the fighting began, a moderation this contributor's colleagues in the diplomatic service attribute rather more to the Commonwealth's own preoccupation with settling and governing the territory it already held from the Louisiana purchase than to any particular restraint of principle.
The war has left remarkably little mark on the Commonwealth's own popular memory, being generally reckoned, where it is reckoned at all, a peripheral episode of frontier administration rather than a war of the first rank, and this contributor, whose own professional interest lies rather more with the European chancelleries than with the Commonwealth's western marches, is content to leave it in the modest proportion the Commonwealth's own memory has already assigned it. (H. LeF.)