As I am redoing my UPCA (Central America) Universe, this is an idea I had to make the whole scenario more likely. I scrapped the idea of Texan California for a more reasonable scenario.
1845 - The Republic of Yucatan was annexed by the Central American Federation, resulting in a Mexican declaration of war and invasion of the peninsula. Obligated by treaty, the Republic of Texas renewed its war on Mexico, sending a second, ARMED group to Santa Fe, taking control of the city and thereby the entire Nuevo Mexico Territory. After taking New Mexico, the Texan forces moved down south and made contact with Rio Bravan revolutionaries to renew their revolt against Santa Anna's Centralist government in Mexico City.
1846 - Central American forces pushed the Mexican army out of Yucatan and Chiapas. The same armed band of Texans arrived in Sonoma, Alta California and incite a rebellion there against the Mexican officials among both the American and Mexican settlers there. With the aide of the Texans, the Californians defeated a small Mexican force and declared their independence, claiming all of Alta California, but effectively controlling only the Northwest section.
1847 - Central American forces progressed northwest into southern Mexico and closer to Mexico city. Texan forces helped the Rio Bravan revolutionaries to regain control over Coahuila, Nuevo Leon, and Tamaulipas, and they supported their claims to Chihuahua and Durango. The state government of Sonora, having been against Santa Anna's Centralist government, declared its independence and established diplomatic relations with Texas, but not California.
1848 - Central American and Texan forces sieged Mexico City until Santa Anna was forced to surrender, ending the war.
-Texas annexed the New Mexico Territory and released its claims north of the Colorado River to California. It signed a mutual defense pact called the "Laredo Pact" with the Republic of the Rio Bravo to guard the two republics against both Mexican and American aggression. After trying to assert its authority over Chihuahua and Durango, the two western states declared their independence and joined the Laredo Pact of their own accord.
-The Californian government in Sonoma sent expeditionary missions to the Mormon settlements around the Great Salt Lake and to southern Alta California to establish control. They had limited success with the Mormons after the Californian expeditionaries offended their beliefs and practices on more than one occasion during their stay. Upon reaching southern Alta California, the Mexican population rejected the Sonoman government as a form of White domination. When they reached Ures, the capital of the newly proclaimed Republic of Sonora, things went well until they began to discuss border claims. Both states claimed Baja California and the Mesilla Valley. Diplomatic relations broke down and the Californian delegation fled back to Sonoma. After Sonoran troops marched into Mexicali, Baja California, a small Californian army was sent to take back the peninsula. They were defeated time and again as they fled north through southern Alta California. They finally pushed back the pursuing Sonoran army outside of Monterrey. In the subsequent peace treaty, all of Alta California south of Monterrey Bay was given to Sonora. Soon after, the Mormons to the east declared their independence as the state of Deseret, and Sonoma was unable to stop them. As Texas did not intervene in favor of California, it and Deseret quickly fell under the influence of the United States and became protectorates.
-Zacatecas was split from Mexico at the end of the war, but it was devastated and its economy hinged on as to the ocean through Mexico. It quickly fell under Mexican influence and a centralist government was established that followed the one in Mexico City. After the war with California, support for the Federalist government in Sonora fell and it was replaced with a Centralist one. A non-aggression pact was signed with Mexico, making it a Mexican alley.
This idea is still kinda rough, so any advice, no matter how harsh, would be much appreciated.