Small map I made based on a WI: Zachary Taylor lives.
In international relations theory there is a concept called "Balancing". Balancing refers to states ganging up to knock down a rising power in fear that said rising power will displace the status quo and become the new hegemon. If you've ever heard of the phrase "Balance of Power", you'll understand balancing. In the Americas, this historically hasn't been an issue- American hegemony was slowly accepted by the powers that be, despite close calls. But the genuine fear of the midcentury of the American Giant was felt by everyone, not just Latin America. And at the turn of the midcentury, tensions were high.
Now historically, it was found that President Taylor's Secretary of State John M. Clayton was found supporting Filibusters in Central America and Haiti (yes that plan was actually made). This caused a rage in France and Britain due to fear of US expansion from the Mexican American war, and tensions remained high throughout the 1850s, with at one point Lord Palmerston considering to send in the gunships. But then Taylor died, Clayton resigned the next day after, the plan didn't go through and treaties were signed with Britain tacitly allowing the Americans to invest more in Central America, then precieved as part of the British sphere of influence.
Suppose the plan isn't found, Taylor lives longer, and Clayton gives support to filibuster operations in Central America and Haiti in 1850. Britain and France are outraged, and war is declared. Balancing begins. The Great Powers- Britain and France, declare war in defense of the empire of Haiti, and the United States is swiftly brought to heel. The war is an easy sell. An insatiable power seeking to expand the ills of slavery. Mexico is brought in for support with promises of debt renegotiation and reconquest.
The actual war is swift and brutal. Despite a sizable population and being the third nation on Earth to start industrialization, the experience and numbers gap is too much. Minie rifles and steam battleships overcame muskets and frigates. Tens of thousands of shipped professional troops destroyed the infant officer corps of the Americans. Inefficiencies stemming from the multiple rail gauges and the policy of "Free Banking" contributed to equipment shortages, funding gaps, and troop delays. And on the fringes, the then marginal groups negotiating for any semblance of autonomy were more than happy to throw in their lot to destabilize the states. Freedmen in particular were eager to seize their freedom when the Europeans granted it. The Seminole were happy to revise their treaties. The Mormons in Deseret hoped they could negotitate legitimacy. And the Comanche, the Comanche profited off war and instability.
Currently is 1852. The Americans have agreed to an armistace. The losses are mounting, the economy is in shambles, the troops haven't been paid. Vast territory has been seized. California has declared independence under Mexican and British mediation all in exchange for some border revisions, open ports, and a nice little constitutional clause preventing any barriers to immigration.
As it is, popular sentiment is that the Whigs were right. War with Mexico was criminal, and the judges have enacted their sentence. Of course, it was worse than a crime- it was a mistake.