Well, this was a lot sooner than even I imagined. Just a heads up, this is probably the trickiest update I'll do in the whole run of this, but I think I managed to get something plausible here. If that's not the case feel free to correct me so I can change it.
Don't worry, everything is smooth sailing from here.
Part 12 – The Short [1] Spanish-American War
Since the Baja California War (known in the North as the Filibuster of 1890) the United States had been trying to regain their honor. The Filibuster had damaged their image in Latin America and in Europe they were seen as unable to control their own people.
For a long time they had seen Cuba, along with the rest of the remaining Spanish possessions, as the way to do so and raise to the level of the European empires. They needed a casus belli, however, and on February 1898 fate would deliver them one when the USS Maine mysteriously exploded and sank near Cuban waters, killing everyone on board [2].
When news of the disaster reached the nation the people called for war against the Spanish, and the government was only too happy to obey. On March 25 the United States declared war on the Spanish Empire [3]. American troops began arriving on the island on April 19, and the war officially began.
For the next two days the American troops dominated their opponents. Despite the courage of some Spanish troops, their men were demoralized, already weakened by the Mexican-led Cuban rebels and overwhelmed by the superior American numbers. For all intents and purposes, the war was already won.
Then Mexico stopped the war.
While it wasn't Mexico by itself that stopped the conflict (though that is what is taught in Mexican and Cuban schools), it was one of the more important nations to do it. Mexico, together with the ABC countries, demanded an armistice when news were leaked about the American intent to annex the island in all but name [4].
By the time they entered the war the Cubans had almost won their independence, and they believed the Americans had entered only to claimed it for themselves. Brazil and Chile especially argued the Americans had sunk their own ship to fake a casus belli. Because of the filibuster against Mexico, many Latin American nations rallied behind this, with some even claiming that “Because they [the United States] couldn't get another bite of Mexico they were now trying to swallow Cuba.”
The outrage in Latin American caught the attention of Europe, and Great Britain offered to lead an investigation on the Maine to clear things up. While the United States declined the Spanish, seeing the writing on the wall, quickly accepted the offer, willing to “give up” (though it wasn't theirs at that point) Cuba if it turned out their forces had destroyed their ship.
The Spanish Empire knew it had lost, even before the American entry to the war. They saw how obsolete their ships were, and how futile their efforts to control their possessions were. It was because of this that they decided to do something dramatic with the rest of their territories before they lost them either to the natives or another country: they offered them to the best bidder.
The Great Auction had begun.
[1] ShortER
[2] Just like OTL, because I'm lazy.
[3] This is a bit different, OTL they waited until April to declare war, and only after sending Spain an ultimatum. Here, because they wanna show they're a strong country they declare war right away.
[4] A early version of the Platt Amendment.