The munitions aboard the Lusitania are improperly handled, resulting in the ship detonating before leaving New York. The loss of American lives results in a fierce backlash against the British, although President Wilson and economics prevent any attempts to go to war with the UK. However, relations are radically chilled, with the United States turning towards a policy of firm armed neutrality. An extensive naval act is passed through Congress, along with funds for the US Army to upgrade its equipment and commit more training and increase its active force size to 500,000 men.
As a result of its military buildup, the US aggressively in responds to events in Mexico in 1916. This overconfidence results in several battles between US and federal Mexican troops. Without concern for the situation in Europe and a strong military advantage, the US presses its issues with the Mexican government by issuing a set of demands. Refusing to accept the American demands, Carranza leaks the list to the Mexican public to stir outrage. Tensions quickly increase until they boil over when some Mexican irregulars raid another American border town, resulting in an American declaration of war.
The US military, already mobilized, is quickly on the offensive against Mexico. The US declaration of war brings a halt to the Mexican revolution, as all of the various factions temporarily put aside their differences to fight the “Yankee Imperialists”. The Mexican regular military proves no match for their US counterparts, but irregulars initially do pose a problem to the Americans due to their attacks on supply lines. Lessons from the relatively recent Moro insurgency in the Philippines are remembered however, and the US soon successfully defeats the threat of Mexican partisan warfare. Meanwhile, the US Navy and Marines seize Veracruz, giving another front from which the US can advance upon Mexico City. After six months of fighting, the US Army surrounds the aforementioned city and engage in a week of urban fighting before the Mexican defense collapse with Mexico’s surrender following soon after.
In the peace treaty, the US establishes a border running along the Yaqui river up the 30th parallel and then following it to the Rio Grande river. Unlike the previous conflict between them, the US does not offer to pay for the territory it takes. Without the ability to continue resistance, the Mexican government is forced to accept the treaty. Soon after the treaty is signed and US forces withdraw to the new border, the remnants of Mexico’s shattered armed forces commence a coup and overthrow the now unpopular government. In the US, however, celebration is the order of the day as America is able add another victory to its belt and, eventually, two more stars to the flag.
In the 1916 elections, Woodrow Wilson is beaten by Charles Hughes largely due to Wilson’s Anglophilia. Hughes pursues a policy of firm, but fair treatment with regards to the new mexican territories. Violent resistance is brutally crushed, but Hughes is firmly adamant that the rights of the rest of the population be respected. A spate of legislation is passed to this regard, with the first authorizing citizenship to any Mexican woman who marries an American and signs an oath of loyalty. All children are granted immediate citizenship as well. These measures work, and over the next few decades the population of the former Mexican territories will come to accept being a part of the United States.