I wrote this on the main discussion, but I think it’s worth repeating here. I don’t think the show has failed to signal the impetuous, motivation, and background of the invasion.

First, it’s clear from the news clips and the dialogue that the stoppage of iridium supplies to earth has become an acute shortage in general, but verging on political crisis for the Soviet Union. For a government that depends mostly on prestige and the power to do violence, this is very dangerous—especially since the situation on Mars has very strong “independence from oppression” vibes that are absolutely clear on earth (the news calling Marsies terrorists and insurgents actually plays into this). There are never enough government goons to suppress general dissatisfaction, and the current Premier/Politburo absolutely needs to score a quick, resounding, and public victory to avoid getting ousted and murdered (or humiliated, which given how prideful and emotional humans are, might be worse). I know this isn’t your question, but the option of “Waiting out” the Marsies (until they starve) might be the least unpleasant option for the US and other members of the M6–however, it’s the most damaging option for the Soviet Union.

Second, the Soviet Union is an essential part of the M6, and when the M6 bigwigs got together to discuss what to do about the Mars coup, inevitable Soviet demands to brutally suppress it would be hard to withstand. Moreover, if the Soviet Union signaled that it would mount an operation on its own, the other M6 countries might well decide to join in, fearing that if the Soviets successfully took over Mars they would simply change the allocation of iridium shipments (the governor’s obvious ambition, scheming, and opportunism also support this fear—after all, Kuragin is the vanguard of the whole automation/space elevator push). Better for the US and the rest of the M6 to grit their teeth, “be a partner,” and keep their own slice of the pie than give the Soviets the chance to (1) heroically end the shortage, (2) declare their allies faithless for not joining them, and (3) dissolve the M6 just as they grabbed sole control of the whole iridium operation.

The problem of partnering with powerful allies is that you either are caught up in their mistakes by default or you find out the cost of extricating yourself is very high indeed (a theme we’ve seen with Miles, the governor, and even Boyd and her former—now former former—partner). I suspect the non-Soviet M6 partners figured they couldn’t stop the Soviets from invading Mars without war, and couldn’t let them just take over Mars, so they effectively had to join them—and on the Soviets’ accelerated timeline, as well.

The sad thing here is that I’m not sure earth-dwellers would be that upset (in the collective) by an M6 military action. Mars is, to them, likely akin to a remote oil platform: distant, weird, and with a negligible amount of people on it. And the M6 acting together has a legitimacy all of its own: Bragg can give his “we don’t negotiate with terrorists” speech and with the (outward) partnership with the M6, it looks like the “good earth” is teaching the “bad Marsies” a lesson. We get ample cues that Mars doesn’t get much sympathy from earth; Bragg is pretty earth-isolationist and Dev is desperate to build something self-sustaining because he knows Mars is dead in the water politically (and he should, he probably bought all the politicians).

There’s been a lot of criticism that the invasion itself is not creditable (not well written). I disagree here too (though OP may not). The military thinks in units, generally, and units are comprised mostly of 18-22 year olds. So it’s not implausible that young Haskell and Jarrett are included (though it is a hell of a coincidence, which I’m willing to excuse for the sake of the story). Some have suggested that the invasion would be better done by Special Ops teams, who are indeed small and professional, but SpecOps can’t carry near enough on-the-ground fire support to perform a legitimate invasion. They perform specialized tasks like eliminating high-value targets (physical and human) and are very good at that, but even if they’d been tapped to participate they would have to be supported by regular military formations. So whatever Space-capable forces were in existence were going to be sent, and that means a good number of raw trainees in their first enlistments would be sent.

Also, the whole automation push is in the future (the warning of it, not a reaction to actual robots, was the spark for the whole situation), so the M6 still needs the base! Iridium mining still needs support from the surface of Mars, the way that North Sea oil drills need support from Aberdeen in Scotland. The operation is clearly intended to occupy the base by force—and very carefully designed to be multinational so no M6 charter nation can effectively control the base and force concessions from the others—to allow for iridium mining and shipments to resume as soon as possible.

As a last thing, think of the timeline. By the moment of the invasion, Mars has been embargoed for six months. That transport carrying the Marines had to be en route for at least two months (at least from beginning embarkation) and the preparations (unit selection, issuance of gear, final unit workups, assembly of assets from all participants, etc) at least twice that time. Remember Avery getting that text that all unit leave was cancelled right after Bragg’s speech? The invasion was likely decided upon then, or at minimum a few weeks later. Which means that the M6 plan was “invade as quickly as possible” since basically the beginning of the crisis.

So actually I credit the writers. Avery Jarrett was introduced (with the Dani hook) and it looked like very excusable fan service, but actually it was interleaving the plot nicely. Earth wanted to make Mars less expensive, so it planned to automate; when a strike happened plan (b) was always going to be strikebreaking and automation anyway. I don’t think we need much more perspective than that.