History repeats itself... until the underlying problem is resolved. For the United States and Mexico, the problem has been a lack of natural land borders allowing for a large, and until very recently, undefended borderland. Following the closing of the border and the mass deportations of the 2080s, Mexico saw their northern borderland flooded with displaced expatriates who were too poor and uneducated to qualify for Mars colonial status. Mexico's economy suffered greatly and began positioning troops on the border to aid with the resettlement of their citizens. The US, already using the National Guard to patrol the South West and relocate people to Mexico, took this as an escalating threat and in turn, increased surveillance and drone flights over Mexico. In 2088, the Mexicanos Libres party was established in the US on behalf of the citizens of the South West who were fortunate enough to avoid deportation, and advocated for the rights of Mexican immigrants who had been living in the US for generations. Everyone was convinced war would break out on the US-Mexican border, which is why what happened next was such a shock, and is largely blamed for perpetuating the underlying problem of US-Mexican relations for nearly forty years. Mexico could not feasibly settle its people in the territory they already controlled, while still maintaining a large and dynamic economy. To resolve this they needed to relocate these undereducated laborers to areas of the country that needed their skills most. Many stayed on the border, but 30% were diverted to Central America and with the stroke of a pen by the Mexican President, the region was incorporated into Mexico as the largest economic zone in the nation's history. Central America had been deep in the Mexican sphere of influence for over a century, to the point that the annexation seemed like a formality, but the US took it as a direct threat to their space elevator in Ecuador, and their trade rout through Panama. The US invaded Panama to secure the Canal Zone, and war broke out almost immediately afterwards.
The fighting was concentrated predominantly in Central America and South America, with a proxy fighting in Africa and the South Pacific. Mexico was believed by the US to be an easy target, but try as they might they could not push them out of Panama. Mexican allies in South America were almost as successful at disrupting US interests, driving the Pacific Fleet of the US Navy to stay fixed around Ecuador for much of the war. The conflict dragged on, with neither side willing to directly strike the other on the US-Mexican border, as the US didn't want to risk loosing the South West, and Mexico didn't want to risk a larger war. The war was fought to a stalemate after only two years, and in 2092 the US and Mexico declared a ceasefire. At the Sao Paulo conference, the US agreed to recognize Mexico's claim to Central America, save for Panama and Belize, which were admitted as states into the Union, while Patagonia and Mapuche were granted independence from Argentina and Chile, after American forces in those countries staged revolutions to further block Mexico's naval capabilities. Neither side really won the war, but shortly after the end of the conflict, Mexico began a policy to more fully integrate its sphere and counter the US in the border lands using a policy of Ciudadano Expatriado (Expatriate Citizens). Mexico created a series of "Virtual States," and allowed people of Mexican decent living abroad to vote for representatives in the Mexican Congress. As the Mexicanos Libres Party evolved into a regionalist party, it quickly became clear that while Mexico did not win the war any more than the US, they have a stronger claim to the South West.
While America and Mexico licked their respective wounds Eurasia and Africa fractured as the majority of their populations clamored to leave Earth to seek new lives on Mars, who's population now stood at over 1 billion citizens. Fueled by the great O'Neil ships that cycle between the two worlds, carrying millions of people at a time, Mars was now the most populated colony in the Sol System. As the de facto capital of the colonies, united by a post-scarcity economic system governed by a network of quantum computers, Mars's standard of living had become the highest in the system, having passed the United States on Earth in 2093. Still they are denied equal citizenship within the United States, satiated only by the Rosalind Amendment, which granted Mars the right to form Territories and send resident commissioners to Congress. Meanwhile, in Earth orbital and Lunar colonies, harassment and terrorism by the Guardiola Foundation continues despite continued protests to Congress and the Mediterranean Union that harbors them, despite public condemnations of the Guardiolas. The Mediterranean nations are, like much of Earth, more concerned with local affairs. Protected by an alliance with Mexico, and prosperous and aristocratic, and does not believe itself threatened by any local power, to say nothing of the American space colonies.
The exodus of people from Earth to Mars was already relieving the strain on a number of powers with surplus populations, however it was also opening up opportunity for division, as the "silent majority," of many nations left, a power vacuum was left open for regionalists and separatist groups. Western Europe was the first to suffer from this, as its well educated, but economically stagnant population fled for the new opportunities of the colonies, leaving their home countries to disintegrate into smaller, bickering nations. Turkey, already weakend from decades of trying to govern many unruly territories, was torn asunder as many of its citizens left to start new lives on Mars and Titan. China saw the largest transfer of citizens to the American space colonies, and despite the power vacuum left by the Turks, they were forced to look inward and secure their borders from separatists groups. The world is breaking apart at the seams, only the Americas seem relatively stable, as two empires look to avoid another major conflict. But the center cannot hold.