The search for personal wealth, as well as overcrowding in the cities, were two big factors for the westward expansion of the United States of America. Many settlers moved west to start their own farms, out on the Great Plains. Others were attracted to rushes of gold and silver that prospectors found out in the old West. The formation of concentrated mining towns began in the 1850s, especially after the Mexican-American War. Many of these mining towns would dry up when their resource did, leaving the ruins of many “ghost towns”. These towns would erode to dust in the wind, monuments to a mad dash for precious metals that ended abruptly with the exhaustion of the resource in question. Not all mining towns became ghost towns—several acquired other industries and became prosperous. Other towns formed near bodies of water due to being more fertile for agriculture. Many agricultural communities sprang up near the rivers of the Great Plains. Both types of community became ethnically diverse due to the large numbers of immigrants going to America, the land of opportunity. Even immigrants from China arrived at some of the gold and silver rushes—although racism was always a problem for them. Mining companies often deceived immigrants with little knowledge of English into working harder and longer hours. White mining workers decried “Chinese Cooliesism” (a racist term for supposed industry preference for Chinese immigrant workers). The US government was originally too preoccupied with the Mexican-American War to particularly do anything, and the issue of white mining workers disagreeing with the companies over Chinese immigration would remain a problem.
A treaty that defined Oklahoma as part of the United States of America and not in “British Columbia”, among other factors (such as more border clarification to avoid conflict). (Note: “British Columbia” in Jefferson’s Anti-Slavery Crisis consists of OTL North Carolina, South Carolina, Georgia, Florida, Alabama, Mississippi, Tennessee, Louisiana, and Arkansas). The rebuilding of “British Columbia” would distract its residents from competition with the United States of America—which was often so bad, an urban legend stated the term “Clean Old-Fashioned Hate” referred to the US—“British Columbia” hatred. That was an incorrect notion; the term had its origins from the lynching of African Americans due to hate groups like the “White Leagues”. This was most common in Kentucky and Virginia, and the federal government attempted to pass an anti-lynching law in 1859. That didn’t work, but it would inspire future attempts. One would eventually succeed in 1868, but by then, many such lynching murders occurred, causing terror among African Americans. A migration of them moved out west, partially due to greater opportunities there, and a belief that the frontier was more accepting of them. That was partially true--although discrimination and racism still existed even in the frontier. Nevertheless, many western communities become multipolar, cosmopolitan, and multicultural places. They personified the "Melting pot" that was America, as they took in excess population from the cities, and various migrations seeking opportunity.
One important industry besides farming and mining was the bison industry. This eventually led to the bison becoming an endangered species, and later becoming one of the first protected species under the Theodore Roosevelt administration far later. The bison was often hunted for its hide. Its meat was also used to supply the workers in the West. The industrial scale of the bison industry led to ecological destruction on the plains since the bison was a key species for the Great Plains ecosystem. "Buffalo Bill" was one of the first people to grow wealthy from the bison industry--and he became one of the personal success stories of the United States of America. Cattle ranching also occurred in the Great Plains, and also in Texas. This industry was limited by the prairie grasses, and was not seen in the Rocky Mountains area. Cattle (usually Texas longhorns) would be herded northwards by the first cowboys (many of which attracted all sorts of legends around them), fattened on the prairie grasses for slaughter, finally slaughtered in slaughterhouses, and turned into meat products to feed the appetites of America. This, too, caused problems for the Plains. The first "Black Blizzard" in northern Texas and in Oklahoma started due to overgrazing of various territories. It damaged the cattle industry for a few years, as once-productive land became dust in the wind.
Westward Expansion had been key to American identity almost since Independence, but now it was becoming even stronger than before. A friendly ally to the north meant that the U.S. claims to the Oregon Territory were no longer disputed. South of the 49th parallel, the boundary between the U.S. and Canada, the opportunities for expansion seemed tremendous. At this time period, there were a group of people called “annexationists” who wanted more land for the United States. Specifically, they were eyeing Mexico’s lands, which seemed full of promise and resources, and controlled by a weak government. They also wanted to have Texas—and there were many Texans who wanted to be a part of the United States of America. Controlling the Kansas--Oklahoma Territory--Texas corridor (note that the US--British Columbian borders were defined until the Mississippi River, leaving the status of Oklahoma Territory in limbo until the U.S. and Britain signed a treaty stating that the U.S. would obtain the Oklahoma territory for $10 million). Mexico, however, would not tolerate Texas becoming a part of the United States of America. The stage would be set for the Mexican-American war if Texas became part of the United States of America—which many believed would be only a matter of time due to the prominence of “annexationists” in the U.S., and the fact that Mexico's government was almost known for its incompetence. Military coups weakened the integrity of the Mexican government, and many Americans eyed "easy prey". The massacre of a group of Texan settlers in disputed territory by Mexican soldiers didn't help any. On July 4, 1852, the Texas government and the American government agreed to let Texas in as a state in the United States of America. This infuriated Mexico, who declared war on the United States of America. The Mexican-American War had begun.