The big thing to remember is this: The Coalition States does not have a war machine; the Coalition States is a war machine. Everything about what the Coalition States is, and how it works, revolves around the ability to wage war. Given its stated objective to liberate Earth for Mankind by way of exterminating all things supernatural and alien, complete with an obsessive (and hypocritical) emphasis on genetic purity (because you can't take up the Nazis as a model without becoming eugenicists), this nigh-omnicidal regime has to be a state engaged in permanent total war in order to accomplish those objectives. This has grave consequences for both the population as well as for the oligarchy ruling that population. I'll save the oligarchy's issues for a future post, and focus instead on the population here.
For players, choosing to play a Coalition soldier means that you choose to play a young man (by default, age 15) who grew up in this environment and believes that this life is normal. Your man cannot read at all, is functionally innumerate (counting fingers and toes is not out of line), learned what he knows solely through the hands-on/on-the-job training he received at the hands of his father/trainer (complimented with cybernetic downloading to the brain), completely believes in the authority of the Emperor and therefore of the States' power over him and his, and he does not question the dogma relentlessly hammered into him by the (state-owned and controlled) media. Your man is a True Believer, and he will think and act as the propaganda programmed him to do, even after suffering the trauma that comes with exposure to combat against those same supernatural and alien entities. (The Army Medical Corps ensures this with psychological maintenance, augmented by cybernetic technologies.) Therefore, to play a Coalition soldier is to play a militant adherent to a cult of personality that masquerades as the government of a free--but beleaguered--Mankind (a.k.a. a personality cult operating as a secular religion, like what happened in France from the Revolution through the fall of Napoleon); the lives experienced that one finds in such conditions will be no different from those of common soldiers in other historical totalitarian, authoritarian states.
Your play experience, whether your man is deployed to the front line or enjoys some time in the rear areas, will also be complicated by an atmosphere of anxiety and paranoia. Your man has heard stories for the whole of his life about the deceptive nature of the enemy and the ruinous powers they employ to destroy Mankind, and they are trained to see any intellectual behaviors outside of approved authorities to be signs of witchcraft or other malign presence and must be exterminated immediately. It's a horror experience in the field, a blend of Call of Cthulhu and All Quiet on the Western Front/Platoon, when done right. (When you fuck it up, you get the movie version of Starship Troopers or one of those low-budget movies like Dog Soldiers.)
At home, this shifts to a blend of Orwell's 1984 with Heller's Catch 22; you can't be honest with those who are close to you, or claim to be there to help you, due to their overriding loyalty to the Emperor and official dogma. (Facts and truth are for the oligarchy; everyone else just does what they're told, how they're told, when they're told like the obedient children that the oligarchy wants them to be- or else.) People who slip up disappear, never to be seen again; this is the hint to stay in line at all costs. The only relief that such severe stress as this imposes comes from an environment that does a hell of a lot to give validity to the official propaganda; there really are a lot of hostile entities out there that want to conquer or destroy/devour Mankind and take Earth for themselves, so killing those that get found out is not necessarily the result of self-serving skull-duggery by the oligarchy, and that produces cathartic release that keeps things in check well enough for the oligarchs.
For Game Masters, choosing to run a Coalition-focused campaign means creating and giving substance to that environment. One would be wise to take time to study the historical examples--not just Nazi Germany, and not just popular or easily-gotten materials; read the academic works--starting with Napoleonic France (what some call the ur-Fascist or proto-Fascist state) and going through the history of the European nation-state's emergence. The environment and paradigm that the Coalition States embodies and exploits far more resembles the examples of 19th and 20th Century European authoritarianism, including the totalitarian states, than anything prior to that historical epoch. It also would be wise to read up on the psychology of authoritarianism, of how cults work, and the practical application of mass psychology--the psychology of real-world socio-political power--because a show of that reality gives a great deal of substance to the fictional environment and greatly aides in the players' ability to fully immerse into their man's world (and, therefore, to think and act as they would).
Emphasize the oppressive mental and social environment that is Coalition culture. Conformity, hierarchy, deference to authority and respect for authority, and just enough examples of hostile entities to make Coalition propaganda seem utterly believable to the uncritical and uneducated Coalition population are the key elements to making a Coalition campaign work as intended. For veterans in particular, actual combat can often be seen as a very comforting release from the otherwise everyday stress of their lives; this dysfunction should be commonplace, compelling intervention by the enforcers (Coalition Army officers, usually Medical Corps or Intelligence operatives, who are not only trained in body language and such but also telepaths.) who are a secret counter-intelligence force. Your campaign alternates between terror and horror as dominant moods and themes, coloring everything else that occurs, so prepare yourself accordingly. (Make good, and at times frequent, use of the Horror Factor rules.)
This is not the sort of gameplay experience that everyone wants out of RIFTS, as it emphasizes the fact that the common Coalition soldier is not in control of his life and does not determine his fate. What powerful technologies he possesses and employs does not translate into actual autonomy or freedom; he is a thrall, as he is enthralled, to a dominant malevolent power that has such a powerful and invisible form of mind control on him that he is unable to even conceive that such power exists- let alone that he is under its control. (And no, I don't mean telepathic domination.) This is the experience of a victim in a horror story, or a fool in a tragedy, but certainly not that of a heroic adventurer seeking fame and fortune or that of the hero taking down the Dark Lord. He is one of millions, completely disposable and therefore expendable, and if he endures long enough he'll be forced to confront the reality of his circumstances- and, most likely, accept both it and agree that this is as good as it gets (and therefore continue to go along with it). A merciful Game Master may conclude such a character's life by transforming him into one of the Emperor's Immortals.
There are other sub-settings that those players should be made aware of, and encouraged towards, instead if this does not appeal to them- Game Masters included. As I said above, I will go into how the oligarchy plays next week.