(I am new to alternate history, mapping, and this forum, so I really appreciate any criticism or feedback that you could give me. I also want some help developing this timeline and scenario. Thanks!)
Fractured America
Dark Blue: Bear Flag Republic
Purple: Mormon State of Utah
Green: Mexico
Orange: Southwest Confederation
Grey: Confederate States of America
Light Blue: United States of America
Timeline
Notes: The Bear Flaggers, in this scenario do not want to join the U.S. They are intent on becoming an independent nation.
May 13th, 1846: United States of America declares war on Mexico.
June 14th, 1846: Bear Flaggers capture Sonoma.
June 15th, 1846: The United States signs the Oregon Treaty with Great Britain, giving them the land of Oregon Country under the 49 parallel.
July 7th, 1846: United States captures Monterey.
July 9th, 1846: United States lands in Yerba Buena (San Francisco), and are met with resistance from the Bear Flaggers who occupy the San Francisco Presidio. The United States had a strength of 247 men in total, along with the USS Portsmouth, while the Bear Flaggers only had around 100 men, most mustered from the nearby towns. The U.S. ship saw a flag other than Mexico’s, but attacked anyway, losing around 24 men. The defenders lost 20 before they surrendered.
July 21st, 1846: People in the neighboring towns begin to hear of the battle, and the ones in support of the Bear Flag Revolt form militias to take back the town of San Francisco.
August 29th, 1846: Militias from around San Francisco swarm into the city, claiming it as their own. The small U.S. garrison is overwhelmed,and surrendered with no casualties.
September 14th, 1846: Militias capture Monterey from the U.S.
October 12th, 1846: The United States learns about the attacks in California, but decides not to do anything yet. They instead focus their forces on Mexico. The Bear Flaggers take the time to reinforce and better train/equip their army.
July 22nd, 1847: The first Mormons arrive in Utah.
August 31st, 1847: The Bear Flaggers attempt to befriend the Mormons, in hopes of creating a western alliance to hold of the United States.
October 20th, 1847: The Mormon State of Utah declares independence from Mexico, with encouragement from the Bear Flag Republic. A large amount of the territory the United States hoped to gain was now taken. However, they wished to annex the new nations peacefully after the Mexican-American War.
February 2nd, 1848: The Mexican-American war ends, and the Wilmot Proviso is included in the treaty, to the disliking of Southerners. An area in the southwestern deserts is now considered disputed territory between the Bear Flag Republic and the United States, as the BFR originally claimed it as when they rebelled against Mexico. However, Mexico gave the land to the United States in the treaty.
March 9th, 1848: The United States attempts to annex both the Bear Flag Republic and the Mormon State of Utah. Both nations refuse the offer. The United States could not take the territory by force either, as they declared independence from Mexico.
June 17th, 1848: The southwestern states leave the Union and create the Southwestern Confederacy. They felt the need to secede because while their states were free, the majority of the population was in support of slavery. Also, many influential and well liked people in the area were arrested for owning slaves, further angering the population. The new country included Arizona, New Mexico, large parts of western Texas, and small parts of southern Nevada.
July 10th, 1848: Marcus Hill becomes the president of the Bear Flag Republic. Strongly against slavery, he has a natural hatred for the newly formed Southwestern Confederacy. He was also an expansionist, looking for opportunities to acquire more land for the nation.
August 17th, 1848: The Bear Flag Republic enters troops into part of the disputed territory, which is now in control of the Southwestern Confederation. This is considered the start of the war.
August 23rd, 1848: The United States declares war on the Southwestern Confederation, wanting to quell the rebellion and restore the Union.
August 24th, 1848: The Bear Flag Republic invades parts of southern Nevada.
October 1st, 1848: Angered by the United States declaring war on the Southwestern Confederation and the imbalance of free and slave states, the south secedes, creating the Confederate States of America.
November 13th, 1848: Mexico forms a secret alliance with the Confederate States of America, both planning to attack the United States. Mexico begins invading through the Southwestern Confederacy, as they do not recognize them as a legitimate country.
November 27th, 1848: The Bear Flag Republic annexes part of the Oregon Country. They stop when met with resistance from American settlers.
December 3rd, 1848: The Confederate States of America begin invading Oklahoma. The large Indian population there begin resisting, as they were being oppressed by the occupying Southerners. The United States declared war on the Confederates after the invasion, and sent troops back into Oklahoma. Oklahoma becomes disputed territory.
December 4th, 1848: Mexico makes land gains in large portions of southern Texas, New Mexico, and Arizona.
December 13th, 1848: Armies from Mexico and the United States meet in a large battle in southern Arizona. Each side takes heavy casualties, and each of their presences are weakened in the Southwestern Confederacy.
December 18th, 1848: The Southwestern Confederacy, holding the line against the Bear Flag Republic, sends some troops to go on the offensive. These troops successfully eliminate the U.S. soldiers in the area, but fail to dislodge the Mexicans, due to their insufficient numbers.
January 2nd, 1849: The United States, leaving the Southwestern Confederacy, sends more men to invade the Confederate States of America.
January 15th, 1849: Requesting assistance from the Mormon State of Utah, the Bear Flag Republic makes a push into the Southwestern Confederacy, but it is ultimately repelled.
February 17th, 1849: The Mormon State of Utah invades the Southwestern Confederacy’s northern border, taking more land in Nevada. Nevada is now entirely split between The BFR and the MSU.
March 29th, 1849: Mexico requests assistance from the Confederate States of America for a final push into New Mexico. The CSA refuses, though, due to their friendship with the Southwestern Confederation. Mexico has insufficient strength to continue without support from the CSA.
April 5th, 1849: Mexico, The Southwestern Confederation, The Mormon State of Utah, and the United States sign the treaty of Santa Fe. The treaty recognizes the independence of the three newly created nations, gives Mexico land in Texas, New Mexico, and Arizona, gives the Bear Flag Republic and the Mormon State of Utah land in Nevada, gives the Bear Flag Republic land in Arizona, and makes the United States recognize the BFR’s annexation of parts of the Oregon Country.