A region of the galaxy is controlled by three human factions.
The Empire of Excellence (faction colors are black and white)
The Alliance of Freedom (faction colors are blue and white)
The Union of Justice (faction colors are red and yellow)
As it follows from their names, they value excellence, freedom and justice the more respectively. But this is often just propaganda (see below).
The three evolved earlier from Domain of Excellence, Federation of Freedom and Republic of Justice, after each faction had their satellite states incorporated.
The Empire of Excellence is not a monarchy. It is governed by the Supreme Council of the Excellent, the highest sort of the citizens. All the population is divided into three classes, and each class in divided into at least three sorts.
The highest class, the Selected, have full citizen and voting rights, but only the first and the second sorts are allowed to occupy management positions. Only the first sort participates in the Supreme Council and makes laws.
The second class are the Tolerated, have restricted rights and no voting privileges. The first sort of the second class has nearly full citizen rights except voting while the third sort has their rights severely limited.
The third class is the Garbage. The state aims at best at as harsh exploitation of them as possible. The first sort is deemed to forced life-long labor at huge state plants, but they are moderately fed and cared about so to serve as long as possible. The second sort is deemed for as harsh labor as possible and upon sickness, eliminated. The life of the third sort is not valued at all, they are used for medical experimentation or for most dangerous and health-damaging works. The factories are mostly owned by big enterprises or state, but private slaves are forbidden.
The division into classes is based on genetics, at least this is what the state propaganda says. POWs usually go into the Garbage class.
The Alliance of Freedom thinks Empire of Excellence and the Union of Justice are tyrants, but trades with them a lot. As the most valuable thing in the Alliance is freedom, a lot of things are allowed. This includes all sorts of sexual perversions and deadly games, including sexual snuff, death fighting, Russian roulette lotteries, as well as (paradoxically) slavery.
One thing is important: a slave is who signed a slave contract, and their children are not slaves themselves, unless they also sign a contract (which is allowed at very early age). Virtually any form of contract is allowed.
The Alliance is composed of several entities which may have quite different laws, headed by the Coordination Assembly.
One of the important sources of the slaves is from trading with the Empire of Excellence. The representatives of the Alliance visit the forced labor plants in the Empire and select slaves for them from the ranks of the Garbage. Formally the Garbage people have to sign a slave contract with the Alliance representatives, but little who refuses. The Alliance propaganda represents such method of obtaining slaves as a voluntary contract and as a mercy for those people, who as it claims, are dreaming of an opportunity to get out from the Empire's plants. Thus they say, they save people's lives by buying them into slavery.
The Union of Justice is governed by the All-Union Congress who are selected on the mixed principles of meritocracy and election. Only those who have decorations and merits can participate in a multi-staged elections into the multiple-level bodies the highest of which is the All-Union Congress, having several thousands of members.
The Union thinks their adversaries are unjust. Upon conquest of the territories belonging to other factions, their elites are often eliminated or put into correctional facilities for prolonged time so to restore the justice.
The Union's propaganda says they are against any kind of slavery but they have a lot of correctional facilities used for forced labor, and anybody supporting practices deemed unjust or disliking the Union's ideology is risking to end up in these places.
Given the above, I wonder what ethical, moral, economic, ideological, political, or legal issues may arise in such a world in the course of peaceful or hostile interaction between the factions.