In comments, Will Baird asks what would have happened to Mexico had the United States taken even more territory in the Mexican-American War.
The answer is that it depends.
First things first: the current U.S.-Mexico border is improbable given the result of the war. The U.S. invaded Mexico hoping to take everything north and east of the Rio Grande, plus California down to somewhere around Monterrey. (Yes, that would have left Los Angeles in Mexico.) The U.S. won all the battles in the north and asked for a settlement. Mexico refused. So eventually General Scott pressed all the way to Mexico.
The American envoy to the Mexican government, Nicholas Trist, was a member of the Whig Party and a vocal opponent of the war. President Polk had appointed him to negotiate with Mexico for precisely that reason; it gave him political cover against accusations that the war was just a land-grab for the slave states. (Northern Mexico was a good place to grow cotton.) The problem was that when the U.S. wound up occupying Mexico City because the Mexicans refused to negotiate, President Polk decided to expand his ambitions. He recalled Trist, intending to demand more Mexican territory. Trist, however, ignored his recall order. In fact, he took it to the Mexicans and said, in effect, “Negotiate with me, you idiots, because the next guy is gonna demand a lot more territory.” The Mexicans agreed with this logic and negotiated a treaty that more-or-less gave us the current border. (Trist was not a bad negotiator: he was authorized to accept a border as far north as Monterrey or as far south as Tijuana; he pushed it as far as he could.) Treaty in hand, Trist returned to Washington, which put President Polk in an impossible position. The Whigs in Congress refused to vote money to continue the occupation of Mexico unless the President accepted a tariff increase.
At the time the Democrats were the party of free trade, which gave Polk three unpalatable choices. (1) Submit the Trist treaty to the Senate and give up on the dream of new slave states in Tamaulipas, Nuevo León, and Coahuila; (2) Get more Mexican territory but accept the permanent tariff increase to finance the occupation for the duration of the negotiations; or (3) get absolutely nothing beyond Texas when Congress refused to finance the army. He went with option (1).
But what if he had taken option (2)? Or what if Trist had obeyed orders and gone home? The U.S would have taken an additional swathe of northern Mexico. How would that have changed Mexican history?
The first question we need to answer is: how much more territory? We know that President Polk told his cabinet as early as May 30, 1846 (a mere 18 days into the war) that he preferred a boundary at the 26th parallel, although he would settle for less.1 When the Mexicans proved incalcitrant, he told his cabinet that he also wanted to seize the port of Tampico and “make the Sierra Madra [sic] the line.”2 That is a pretty concrete counterfactual, and it provides us with a new border. It would run from a little north of Los Mochis, continue east to the north of Torreón and Saltillo. Between Saltillo and Monterrey it would turn south, skirting to the west of Ciudad Victoria and then darting to the coast right below Tampico. (It’s the thick black line below.)
What effect would the new border have on Mexico? My first answer was that it would have nothing predictable until 1910. And upon consideration, that answer is probably right.
Some history: France invaded Mexico in late 1861, ostensibly over unpaid debts. In reality, the French had struck a deal with disgruntled Conservatives in Mexico, who were angered by the anti-clericalism of the Second Mexican Republic. In our world, the French pushed the Republican forces under Benito Juárez back to El Paso de Norte. (El Paso de Norte is now called Ciudad Juárez.) They then established the Second Mexican Empire under Maximilian. But they were reluctant to obliterate Juárez’s forces so close to the United States. A year later, with the Civil War over, the U.S. began to give military support to Juárez. Slowly but surely, the Republicans pushed south, and by 1867 the Second Mexican Empire was no more.
The red arrows on the map represent the movements of the Juárez government. It retreated across the Coahuila state line at the end of 1863. In our history, Juárez took refuge in El Paso de Norte. That city bordered Union-controlled territory, at least once troops from California occupied West Texas in 1862.
My first thought was that the new border would have little effect on the course of the Intervention. The French would be reluctant to attack a Saltillo so near the U.S. border. Either that or Juárez would just cross into the U.S. and establish a government-in-exile. The Republicans would regroup, reinvade and win after 1865 just like they did in real history.
Except it isnt that simple. Is Chihuahua in Confederate or Union hands in early 1864? Coahuila is deep in Confederate territory. (In the real world, the state’s Mexican governor supported the Confederacy.) The Confederacy supported the French. French troops would have cornered Juárez in Saltillo and destroyed the remains of his government, although the president himself might still escape to Union territory.
But Chihuahua would have been in Union territory. First, Coahuila might have been Confederate, but it would have been weakly Confederate. There is no drive on Chihuahua the way the Texans idiotically tried to invade New Mexico. Second, Chihuahua was filled with old Spanish military colonists who would have been fairly aggressively Union when the other option was dominated by slaveowning Protestants. Third, the main travel routes ran north-south, not east-west; the Californians that reconqured El Paso would have found it very easy to travel to Chihuahua City if need be.
So Juárez retreats across the U.S. border in this world, regroups, and reinvades after 1865. Would there be differences? Yes, probably. Would they be predictable? No. In short, losing the northern territories would have changed Mexican history, but until 1910 those changes would not have been predictable or systematic.
And after 1910? The collapse of the Porfiriato in 1910 and the rise of the subsequent regime was almost entirely a northern story. Take Sonora, Chihuahua and Coahuila out of the federation and you remove both the factions and personalities that drove the Mexican Revolution. So you either get no Mexican Revolution or an unrecognizable one.
At that point it’s a mug’s game to predict the result. But it won’t look like modern Mexico. You'll still have a big semi-industrialized country — we are not talking Greater Guatemala. A succession of unstable military governments is the most likely outcome, but not the only one. All we can say is that Mexico would have been very different.
And the United States? I leave that as an exercise for the reader.