1820-1830: Mormonism takes off at a faster and more successful rate in this timeline, more than triple OTL amount of people become followers, and this will spread the amount of followers and make the religion more common, though still majorly unpopular to other sects of Christianity.
1840: The Brooklyn lands in northern California and its Mormons passengers establish a settlement on the coast named Brooks in honor of the ship which brought them there.
1842: The Mormon Exodus of 162,000* people arrive around the Great Salt Lake, and begin to establish a new series of settlements there, with some settling in the west in OTL Nevada, where roughly 30,000 people settled. Near the Salt Lake 63,000 settled, while 37,000 settled in the area of Colorado, and the remaining 32,000 continue the trek west, making contact with the settlement of Brooks, which has grown to about 327 people by then. The massive amount of people merge with Brooks, and the people get along fondly, with the leadership of the new settlers largely taking over the legislative power in the town, while the Mayor is one of the original settlers, named Joseph Edwards.
1843: Contact between Denver (the Colorado settlement), Salt Lake (the Salt Lake settlement), Elden (the Nevada settlement), and Brooks begins to develop into a trading network, with roads and other routes beginning to mature and become more commonly traversed. More settlers from the United States settle in Colorado, establishing three new settlements, Nauvoo, Smolone, and Beno, with the population being roughly 3,000 for each of the settlements, with Nauvoo having a larger population of about 5,600 people.
1844: The remaining Mormon populace, having received letters from friends and family in the western Mormon territories, and hearing of the opportunities west, a vast majority sell everything and trek west, settling several new towns in Nevada, Utah, and California, boosting the Mormon populace, though some groups will not arrive until early next year.
1845: The remaining Mormon settlers arrive in the western lands, and join the now over four dozen towns and villages across the region. Following their arrival it is agreed upon by the regions leaders to meet in Salt Lake City, and form a new, federal type setup of government to manage the new nation of ”Deseret”. A constitution is soon written within a few weeks, largely modeled after the US Constitution, though Mormonism is expressly allowed and protected in it, and religious tolerance is enshrined in the new document. The legislature, called the National Assembly, with two houses, with 3 Senators per province, of which there are currently eight (Colorado, California, Brooks, Nauvoo, Utah, Salt Lake, Nevada, and Rena), with the second house, called the House of Electors, based on population. Deseret’s new government is put into effect almost immediately, and it is agreed to have a tripart capital, with the Grand Court to be in Brooks, the National Assembly in Salt Lake, and the President in Nauvoo.
With the outbreak of the Mexican American War, and the large amount of Mormons in northern California, the breakaway American settled Republic of California is restricted to south of a bit north of San Francisco, stretching to the tip of Baja California. Meanwhile the United States forces are a bit slower, meeting stronger Indian resistance due to the Mormon presence a bit farther west, and some were pushed east. Due to this the American invasion is blunted and delayed, as winter comes down a bit early blocking much of an advance in the northern areas, though the American invasion of southern Mexico in Veracruz and Texas go on schedule.
1846: The Californians, having not linked with any main American force, begins to enjoy the taste of independence, and when US forces arrive in the west, decline to join the Union, instead agreeing to join them in alliance against Mexico, occupying the lands south of Deseret, agreeing to recognize Deseret as a temporary action until Mexico is subdued. Deseret declares neutrality in the war, though it opens relations with California to the south, and both recognize each other, initiating trade.
Word reaches Deseret that Britain, Brazil, Spain, and Portugal have all extended recognition to the new nation, asking to open trade and relations. 1847: The United States Army reaches the army from Veracruz, occupying everything north of Tampico down the river and from there a rough line to Guadalajara and from there to Puerto Vallarta. The occupied territories are put under military rule, most of it being brutal and harsh due to President Polk’s new orders (being nearly assassinated in 1845 by a Mexican man from Veracruz, which turned Polk bitterly against Mexico and seeking revenge, as he lost his lower leg due to the bullet).
With the harsh treatment of Mexicans coming to be heard in California, it turns public opinion against the United States almost overnight, and though Californians don’t love Mexicans the treatment is overly harsh even for them. California pulls its forces from Mexico, seizing some land in Sonora in order to seize full control of the Colorado River delta, as well as seizing a large portion of land west of the Rio Grande, as Texas is not so quick to accept annexation in this timeline, allowing for the Californian seizure of land. The President of California, Peter H Burnett encourages settlement by southern Americans in the region, so long as they agree to live as Californian citizens, and as slavery is in no danger here, hundreds of families will migrate here within the following years.
1848: With the backing out of California and its seizure of a large amount of land, the United States is enraged, and orders the embargo of the new republic. In order to make up for the loss, through words of encouragement (aka threats of force), Texas votes to join the Union, giving it all the land east of the Rio Grande, excluding that claimed by Deseret.
In the war with Mexico itself, the more occupied US Army is defeated when it attempts to take Mexico City, as several generals were either killed in battle due to stronger resistance, or are still far behind lines dealing with rebels. A Commander Robert E Lee will soon rise to prominence in dealing with the rebels, as instead of executing them like many troops call for, he allows them to live, and places his subjugated zones under lighter restriction, thus leading to the Mexican peoples there to see him as a better breed of American, causing the government in Washington to take notice, promoting him to General in August.
He is given command of the forces in Mexico, and he decides to go into the city himself, recognizing his bravery, the Mexican President agrees, and negotiations begin. The talks begin with Lee asking for Mexican recognition of Deseret and California, as he felt the US recognition of the two states meant the Mexicans should as well. His other terms were that Mexico will recognize American annexation of Texas and everything north of and including Sonora and Chihuahua, and in return the US would pay 16 million dollars. After some haggling on the amount of money (which is increased to 17.6 million), the Treaty of Guadalajara is signed, and the United States officially annexes the new territories, giving it a port in the Gulf of California and two options for a transcontinental railroad. Construction of the southern route will be an issue for some time until the resistance there is finally put down.