Mexico annexed after 1848

[True Federalist (진정한 연방 주의자)]

With a territorial government of Baja California as well as Alto California, when California is admitted it is much smaller with the portion south of 36-30N placed in the Territory of Baja California and the remaining portion east of 120W placed in Utah Territrory. Texas is also slightly smaller, with the panhandle being part of the unorganized territory and the portion west of the Pecos being part of Chihuahua Territory.  Eventually 16 States will be formed from what is now Mexico, plus we get two states out of California, and the Southwestern boundaries are somewhat different from what we know.  The Mexican States (as a condition of statehood during the 19th century) by and large admitted as slave states, but save for the Rio Grande states of New Mexico, Chihuahua, Coahuila, and Tamaulipas (which includes our Nuevo Leon) it doesn't take root.  However, while the other Mexican states don't have widespread slavery, save for the Colorado River states of Utah, Nevada, Arizona, Baja California (our Southern California), Cortez (Our  Baja California), and Sonora, none abolish it during the 19th century.  (The 18th amendment provides for a gradual emancipation after it is adopted in 1924.)

Enthusiasm for additional territory wanes during the mid part of the 19th century as the South realises less gain from the annexation of Mexico than it had expected, so Guatemala ends up keeping Chiapas instead of losing it to Mexico.

With Mexican labor readily availble to open up the West, the Chinese exclusion laws get passed in the late 1860's or early 1870's, at least a full decade earlier than they did.  They are also far more restrictive.  The result is that Chinese emmigration to Noth America is confined mainly to the Canadian provinces of Vancouver Island and British Columbia.

Prompted in part by anti-Catholic sentiment and in part by a desire to Anglify the Mexicans, hordes of Protestant missionaries decend upon Mexico in the latter half of the 19th century and far fewer American missionaries go abroad.  By 1900, less than half of all Mexican-Americans are practicing Catholics, and by 2000, less than a quarter are. Thanks to the infuence of the converted Mexicans, National Prohibition (the 14th Amendment) starts sooner and lasts longer before it is abandoned after World War II.

A more inward-looking United States stays out of World War I, but worries about Japanese exapansionism cause it to participate in the sequel, with the U.S acquiring Hawaii and Canada acquiring Alaska from the Japanese.  (The Chinese also regain the ancestral Manchurian lands of their imperial dynasty from the Japanese.)

After World War II, The victorious Trilateral Powers of the United States, the United Kingdom, and Germany set up the League of Nations to try to avoid the scourge of war.  Unfortunately, war weary Russia and Brazil succumb to Luxembougian revolutions, and a Cold War soon settles in between the Trilateral Powers and the Triumvirate Powers of Russia, Brazil, and the Ottoman Empire (which had stayed neutral in WWII)  The Ottomans are essentially riding the tiger, allying with Brazil and Russia solely for reasons of realpolitik and when the uneasy Russo-Turkish alliance of convienence crumbles in 1956, Constantinople, Moscow, and seventy-three other Russian and Ottoman cities are scourged with atomic fire. Besides direct damage, the Eurasian continent suffers from the fallout, both nuclear and political of the war and the United States narrows it focus from the world to just the Americas and in particular Brzail.  Brazil had no nukes, but the Brazilian War bogs down into a protracted guerrilla war for the United States that lasted almost two decades before the last troops were withdrawn.

The United States has offered mostly moral support to the United Kingdom after the 9/11 attacks of November 9, 2001 which saw a Boeing 747 crash into Parliment during the Queen's Speech, despite calls from some quarters to actively participate in trying to establish democracy in Arabia, and concern that prominent Black Islamists such as Mohammed King Jr. have ties to Al-Qaeda.

[True Federalist (진정한 연방 주의자)]

In colonial times Chiapas was part of the Captaincy General of Guatemala. Only the westernmost part of Chiapas was part of Mexico from independence.  The easternmost portions weren't annexed until the 1880's. Most was unilaterally annexed by Mexico in 1842 following the collapse of the Central American Federation, but rather than trying to enforce that claim, I think that a United States struggling with integrating and occupying Mexico would try to simplify things by ignoring the claim or negotiating it away when settling the border with Guatemala.

Vera Cruz may have cotton, but it was already well settled by the the 1840's.  Why bother importing negros when you can exploit the locals?  As for New Mexico and Chihuahua, you can use slaves for things other than cotton growing and those territories were sparsely settled and would have been populated mainly by Southern interests.  The Colorado River states were also sparsely settled, but would be heavily Mormon influenced and the Mormons weren't much interested in having either slave or free negroes living among them at that time.  A good deal of antebellum Northern anti-slavery sentiment wasn't due to being against slavery per se, they just didn't want any blacks around, and having no slaves greatly reduced the numbers of blacks.

With the United States not only remaining a slave owning country, but expanding the area it was used in, I think slavery is likely to persist in the U.S. until the mechanical cotton picker makes it practical for cotton plantations to replace their field hands.  Even so, I'm having to assume that with a South undevastated by a civil war, the cotton picker gets developed a couple decades earlier than in reality.  The only other reason to assume that slavery dies out as fast as 1889 would be the influence of the Third Great Awakening, but considering that the Second failed to do so, I doubt if the Third would.

You underestimate the power and attraction of religion.  With Mexico part of the United States, I see no reason that it would not be affected by the Third Great Awakening of the late 19th century.  If anything, it might be more receptive.