As the struggle between Mexico and the US threatened to explode into a far worse war than any in this century knew, the colonies feared that such a conflict might spread to them; the specter of the Guardiola Incident continued to haunt the minds of the colonists. Mexico had been steadily weaponizing their Orbitals since 2115, and it was no secret that Mexico wanted a piece of the colonies for themselves. This gave fuel to the long dormant Independence movement in the colonies, particularly the Mars Society for Independence and Freedom, which began a more aggressive and often militant push for a formal separation from the United States on Earth. The MSIF argued that only as an independent power could Mars and the rest of the colonies hope to defend themselves, and achieve the rights that the US on Earth had long denied them. They were lead by Maisie Luong, a natural born Martian, and veteran of the Guardiola Incident. Luong rallied her supporters across the planet through propaganda and marches on territory Capitols, often clashing with local police forces. As the Mexicanos Libres Party on Earth continued their tirade against the colonies, and Mexico conducted more drills with their Orbitals, the MSIF got louder and more aggressive. It all came to a head when Luong and her supporters stormed the Headquarters of the ITC in Bradbury. Luong hoped to overthrow the ITC, which she and her supporters had come to see as lap dogs of the US government on Earth. Flying red flags and brandishing old weapons and armor from the Guardiola Incident, five thousand MSIF stalwarts clashed with the police, the territorial militia, and finally the Mars Planetary Guard before the mob was put down. The majority of the MSIF was arrested or killed during the fighting and raids on their safe-houses following the Bradbury Riots; Luong herself was killed during one such raid. While the MSIF was destroyed, Luongist supporters from Tharsis to Araby have continued to fly the red flag against the ITC and the US on Earth, a constant reminder of the tensions that threaten to tear the Union apart.
In contrast to the MSIF, and the Independence Movement, the majority of the colonists have renewed calls for full integration into the Union, flooding the Congress of with pro-integration Resident Commissioners. ITC Chairman, Lionel "Leo" Halvidar has been the driving force behind this renewed call to action. A longtime advocate for integration, and one of the most active Chairmen in living memory, Halvidar was seen by most as a moderate between the revolutionaries in the colonies, and the established forces on Earth. The Levin Administration, having lashed itself to Mexicanos Libres party leader David Castillo, could do little to answer these calls, being paralyzed with fear over a civil war in the Homeland. Halvidar lobbied the government on Earth with one message, "Bring the colonies into the Union, or you will face TWO rebellions." With the parties busy tearing themselves apart over the 2130 midterms, it doesn't seem likely that this warning will be heeded by the leadership of Earth.
While some in the colonies feared being pulled into a war on Earth, others colonial leaders believed that even if war did not spread to the colonies, Earth would be devastated by a conflict between Mexico and the US, and the colonies would be left to manage untold waves of refugees. With Mars's population reaching its cap, and Venus still not fully Terraformed, this was simply not an option. To prepare for the possibility of devastation on Earth, or outright war, the ITC quietly funded the construction of a series of ruggedized, high speed spacecraft in the Asteroid Belt. They would not be outfitted with weapons, and be crewed only for running relief efforts to the Earth, but the ITC knew they were building a fleet to either end a war on Earth, or at least prevent one on Mars.
The Intermarium and Mexico's empire are allies in all but name at this point. Everyone knows another war between Mexico and the US is coming, its just a question of when; but a lot of countries are betting on Mexico to win, simply because they've managed to chip away at the American's sphere over the decades like no other power. The four powers are now down to Three. One Dominant power, and two challengers.
There are some O'Neil type colonies at teh Lagrange points, mostly Mexican, the US builds lots of smaller, more utilitarian platforms. Then there are the great O'Neil Ships built out of asteroids in the Belt and the outer system. They don't have a population like the Mexican colonies, but instead serve as transport ships for people, biomass, and raw materials. Only a couple have large cities inside them, but they're usually the ones transporting people for colonization, so they're mostly deserted until Venus is complete. There are some habitats in orbit around Venus and Mars for terraforming groups, but the biggest Habs are around the outer planets, siphoning off Hydrogen and Helium-3 for spacecraft fuel.
Lastly, would a war between the US and Mexico go nuclear?
Mexico has fewer platforms, but their Orbitals are enormous and very well armed. So long as the colonies are kept out of the war, Mexico is a credible threat in space.
If the US never formally annexes the Anglosphere, they will become so integrated into our sphere of influence, that they'll functionally become vassal states. At that point, it would almost be better if they were annexed, at least then they'd get a say in policy decisions.
Of course the war would go nuclear, but not in the way you're probably thinking.