It began ironically with American settlers in Texas. Then they proclaimed their independence and a conflict went on, Texas winning its independence. Then the Texans joined the US and the conflict continued, this time the American forces occupied California, Monterrey, and from Veracuz, Mexico City. Long distances were covered by both armies, often in inhospitable desert like conditions. The US had 21 million as opposed to 8 million Mexicans. It was politically more stable and it had an industrial/manufacturing edge. Even then, at that time, these advantages were not clear, and it seems many thought Mexico would have won. The US also had a powerful Navy, which blockaded Mexico, and operated both in the Atlantic and in the Pacific.
Ironically again, what began with Americans settling in then Mexican lands has reversed back. Texas, California and New Mexico are becoming ethnically Mexican again:
A map showing the military operations:
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Marcadores