Eltab
Lord of the Hidden Layer
At the outset of the war, the two sides' armies meet in bloody clashes which turn into stalemates of exhaustion. As both governments train a wave of new recruits, they also put out feelers to parties of adventurers to undertake missions which might help tilt the balance. This puts a natural framework around the campaign - after an in-game year, the recruits will be ready for combat, and the adventurers will no longer be needed (the governments think).
Do note that getting caught by the authorities is not going to lead to the typical 'but I can get away with it!' but being hunted down for treason, sedition, or interfering in foreign affairs.
As for the underlying reason for the conflict: read some of the pro-slavery justifications from the 1850s. A few declared that the new industrial worker class would be better off with a Master and an Overseer than trying to make it on their own. In your world, a hothead proclaimed that kind of sentiment from the rooftops, among whatever class of people the author was convinced were naturally incapable of taking care of themselves. Of course, the intended targets will have nothing to do with the advocates ever gaining the power to "make it so."
To avoid a long derailing discussion, do not use they-are-just-a-property slavery, but a system of discrimination and legally-allowed abuse that the target group cannot effectively escape. It might be workable at a table to manifest this as "debt slavery" or indenture-for-life. Read Banker To The Poor to see what I am thinking of, and one reformer's potential solution. Athas-setting adventures might provide you inspiration also.
