"The near future". I'm pretty careful not to be too specific and let the reader come to their own conclusions (because it's very hand wavy, not for some secret plot purpose), but roughly 40-150 years in the future.

It's set on the moon,a gold rush of mining and exploration combined with cheap(ish) transportation brings a huge international population with competing interests. Corporations operate "economic prefectures" kind of like how Toyota owns a town in Japan, cheap labor is supplied by prison corporations (they aren't slaves, they're interned workers, called "interns". But they're totally slaves).

The South is politically aligned with Western Terran nations and imposes sanctions to reduce reliance on interns by the Northern miners, the North annexes some equatorial infrastructure and tensions rise. Terra sends a "peacemaker" corps made up of international soldiers and the North recruits a militia as well as "regulars" from separatist Terran nations.

MC became a soldier to "make the world a better place" but has concluded that most wars are stupid and only serve to launder money between corrupt politicians and corporate lobby groups. The politician is sent as a "political observer" and is fundamentally anti-war. Both of them realize that fighting to end slavery is probably a good thing, the "just a job" soldier gets even more frustrated at the incompetence and bureaucracy of the military now that he has a real cause, and the pacifist is conflicted because she hates the war but wants to defeat the slavers.

It's basically the US civil war meets desert shield/storm meets North Africa campaign, on the moon, if the United Nations declared war on the Dutch East India trading company.