Mars would lose atmosphere to space. It has in the past, and it would again if you gave it one tomorrow. A combination of its low gravity and lack of magnetosphere means that long-term, it can only hold onto the thin sleeve of air it currently has. The rest gets burned away by the solar wind.

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Panel 1: Maida: "...But if you turn just some of the towers off, the Lowlands will be fine while the Highlands eventually suffocate." Close-up of the mural showing towers producing less oxygen as they go uphill.
Panel 2: Maida: "There was no way the Highlands could win when the Lowlanders had the power to turn off the air." Even closer look at the last oxygen tower, with only an image of a skull emerging from the top.
Panel 3: Maida and Tahmaseb continue their walk, emerging from the underpass. They pass more graffiti, including a painting of a shark, and a note that says, "Si no hay pan, para el pobre, no habra paz para el rico." Tahmaseb: "Oh my God." Maida: "We were caught between the side that would let us choke to death, and the side that wanted to burn the Solar System with "seven suns" to make way for the final Buddha, or something."
Panel 4: Maida: "The Civil War wouldn't have started at all, of course, except for the Twelfth Crisis." Tahmaseb: "The Twelff... Co'erence Crisis? Twenty years ago?"
Panel 5: They walk past a small, quiet Buddhist shrine. Maida: "Yeah. All of Mars used to be run by the Old Soviet before the last Coherence Crisis."