It comes from the "jobs that Americans won't do" contention.
American troops establish a cordon sanitaire along the boundary with Mexico with the admonition that any crossing will be dealt with through deadly force. Families are split, lovers are separated and the economy in Texas, Arizona, New Mexico and Southern California tanks.
The Mexican government appeals for help from around the world to help them assimilate the returnees and help does pour in. This is necessary to replace the money sent to Mexico by those laborers inside the US. China in particular gets a real chuckle out of helping out. They request military bases in return for their help a request Mexico quickly agrees with.
Meanwhile back in America, landscaping comes to an end and neighborhoods are awash in weeds and uncollected yard waste. Garbage also piles up and the pristine suburbs become rank and disgusting. Construction slows down and produce prices skyrocket.
The border states see the improvements in Mexico and the destruction of their own economies and begin sub rosa discussions with the Mexican government about leaving the Union and joining the Mexican Republic. China stops buying American debt in favor of Mexican debt.
After several months of standoff the Mexican government approaches the US with a proposal. The "idled" workers can be rented by the US at a rate far above the current cost as long as the workers are permitted to bring their families. The US is close to defaulting on its debt, the ruling party (the Christian Libertarians maybe?) is losing its hold on the public, especially the all important suburbs (now overgrown, covered in trash and full of obese, vitamin deficient women and children complaining that they have to "clean the house", something they really don't know how to do. The President, however, has one more card up his sleeve, an agreement for Russia to buy the bonds using petrodollars coming from the last significant oil reserves in the Caspian Sea.
The climax comes in a tense negotiation between the Mexians and the Christian Libertarians. It seems as though the Russian gambit will work until the announcement of the Four Star Republic (Texas, Arizona, New Mexico and SoCal) is made. Mexico immediately recognizes the Republic, the Russians pull out of the deal and the US Army, now isolated hundreds of miles inside a suddently unfriendly foreign country, pulls back to a line of San Francisco, Denver, St. Louis and New Orleans but runs out of gas halfway and is interred by the Four Star Republic.
The final peace agreement has the Four Star Republic joining the Mexican Republic and the US appealing to Canada for increased access to oil shale. The ceremonies are held in the Alamo.