With the mass deportations of the 2080s and the military patrols of their borders that were put in place during the Second Mexican-American War, animosity between the majority Mexican Southwestern States and the rest of the country began to grow and fester during the interwar period of the early 22nd Century. The Mexicanos Libres party formed out of this animosity and won majorities in most state governments in the region, while controlling the majority of congressional and senate seats from the Southwest as well. The MLP was normally seen as a regionalist party, not advocating secession, rather a degree of separation. A radical offshoot of the party was the Aztlan Autonomous Movement, which did seek to break off from the United States and form either an independent nation or rejoin Mexico. It was this movement that led the call for independence when Lionel Halvidar was elected President in 2132. The state governments of Arizona, New Mexico, Texas, and California, all with overwhelming majorities of citizens with Mexican ancestry, all voted to secede from the Union within months of Halvidar's election, forming the United States of Aztlan. The new country quickly established a presidency under the leadership of former Vice President of the United States, David Castillo, and broke up a number of larger states into smaller states that could be more fairly represented by the new government in Santa Fe.
The rebel states claimed that they owned all bases and other federal buildings in the Southwest, and ordered that US forces on the Mexican border be expelled from their territory. On 2 February 2133 the secessionists attacked the garrison at Camp Navajo in Arizona. Union forces from the border were diverted to defend the base, but after a 9 hour fight, the rebels accepted the surrender of the base commander, and the border guards were captured or picked off before they could return to their own garrisons. Following the attack, President Halvidar nationalized the national guards of the Union states, and called for a buildup of domestic military forces. Shortly after this call to arms was issued, three more southwestern states, and Panama joined with Aztlan instead of supplying forces to fight the rebels. The US Navy then organized a blockade of the California and Texas coastlines, and issued an ultimatum to Mexico and the Latin American Union, that any effort to support the rebels would be considered an act of war.
Despite this early showing of support, Aztlan quickly faced setbacks along its periphery. The National Guards of Oklahoma and Colorado remained loyal to the Union, while Eastern Texas subsequently seceded from Texas, joining the Union as the two new states of Brazos and Jacinto. By the end of 2133, Colorado and Oklahoma were effectively under Union control, with Aztlan state governments in exile. The US invaded the rebel states on February 22, 2133, moving through Northern California, Brazos, and Oklahoma. The US saw victories in the Aztlan state of Shasta and Northern Nevada rather quickly, retaking critical military targets, while forces in Texas found the Aztlan rebels more willing to fight than they'd expected. US Space Marines were deployed to support the Army in Texas and California and with momentum in New Mexico stalled, President Halvidar approved the use of orbital weapons on certain Aztlan targets. By the end of 2133, the rebel forces had been forced to relocate their capitol to Phoenix, Arizona. Utah, an autonomous state in Aztlan, went so far as to sue for a separate peace on May 11, 2134, allowing the US to surround the remaining Aztlan forces.
Mexico initially attempted to remain neutral, but militant elements within the Mexican government pushed to engage the US via proxy conflicts around the planet, to give the rebels a fighting chance. By the end of 2134 the US had successfully recaptured much of Texas and California, and were nearing the Aztlan capital in Phoenix. The conflict now threatened to spill across the border into Mexico, as northern Mexicans, many of whom were once US citizens, began actively supporting Aztlan's bid for independence. In some cases, Mexican volunteer regiments joined up with the Aztlan Army. At this point Deandre Garcia lost her bid for a second term in Mexico, and was replaced by the more radical Edwin Jaso's administration, which began quietly sending arms to the rebels. The Jaso administration began directly engaging the US across the planet, while supporting the Azlan rebels with weapons and intelligence. Some of the more radical rebels conducted public executions of all those who spoke out against the rebellion or stood in their war. This ultimately led to greater escalation in the conflict, as US forces were recalled from engagements in Africa and Eurasia to put down the rebellion at home.