WI: Mexico is annexed by the United States

I don't see how given it's positing Mexico has a Second Civil War in the 1980s and then gets annexed by the United States.

Probably quite a few, even some on the hard right were open to it back in the Bush years.

With regards to the possibility of this on the American end, I think there is equal validity. The Annexation of Mexico by left wing author John Ross claims the Reagan Administration sought to purchase Baja California from Mexico in the 1980s. Whether or not that's true I leave for the reader to decide, but there definitely has been agitation on the Right to annex Mexico, even to the present in some form. U.S. officials were also open to the prospect of some degree of economic and political unification during the Bush II administration according to Wikileaks documents from 2011. The 1990s were also a state of flux. The Jack Kemp types were still around and Bush II would kinda tap into that in the 2000s, with "Compassionate Conservatism" and famously winning nearly half of the Hispanic vote in 2004; all the more interesting because the U.S. population of Mexicans tends to be more Left wing than native Mexicans and Mexican-Americans compromised a very large share of the U.S. Hispanic population then. 

Mexico has a tax to GDP ratio of 16.2% as of 2017, as compared to 27.1% for the United States. Given Mexico would likely be brought under the same regulatory burden as the United States, applying the aforementioned U.S. rate to their OTL 2019 GDP of $1.15 Trillion results in ~$312 Billion in revenue gained. To put that into perspective, the entire cost of the refundable part of the Earned Income Tax Credit ($55 billion), Temporary Assistance for Needy Families ($21 billion), Supplemental Security Income ($43.7 billion), food stamps ($75 billion), and housing vouchers ($18 billion) and the Child Tax Credit all together cost just 212 billion dollars. Granted, Medicaid might pose a problem, but that's quite possibly largely made up for by the fact that the median age of Mexico's population is 27.7 years (2015) while in the United States it's 38.2 years; in other words, Social Security and Medicare probably is made solvent by the influx of new workers. If that's not enough, it'll probably encourage the United States to start making reforms done like in other parts of OCED, in particular Singapore.

Meanwhile, on the U.S. side they've just added the 12th largest economy by industrial output, which outpaces other nations such as Australia and Russia while is closing in on being the same size as France and Italy in terms of GDP (PPP) even IOTL. A sustained investment of $200 Billion over 10 years could get the newly annexed Mexico to a modern infrastructure system, while U.S. education reforms could get it up to Spain/Italy levels in terms of education. Add in U.S. law preventing the Drug Wars and reducing corruption, and an ATL Mexican economy 30-50% larger is certainly in the ballpark by ATL 2019.

Using the existing U.S. tax code to Mexico would net about $400 Billion, enough to cover the social services needed given the U.S. spends about $750 Billion on a nation of 330 Million. As for law order, I think the United States could handle it. The reason corruption and crime is so pervasive is a function of the existing political structures, but also due to the fact Mexico only spends 1.4% of its GDP on law and order; the average in OCED nations is 4.7%. Italy, which had to tackle its own issues in terms of corruption and organized crime, spends 6.2% of its GDP on law enforcement. The U.N. Office on Drugs and Crime (UNODC) found that, per 100,000 people, Mexico had just 6.8 prosecutors and 4.2 judges in 2013. This is compared to Italy with 3.1 prosecutors and 17.1 judges. Adding to this issue is the fact that between 2006 and 2015, the number of Federal police increased from 11,663 to 43,724 but real wages only rose 9%.

A Greater Serbia will not happen all because Mexico is annexed by the United States.

Up until U.S. involvement they were winning, thanks to Belgrade retaining the stocks of the old Yugoslav Army while Croatia, under the same arms embargo as Serbia, was unable to sustain its forces. American involvement was also critical to foraging a European coalition to get involved in the situation in the first place.