So Kelsea has now learned that her mother bought peace with the Red Queen by promising her 150 slaves every month. She learned that the hard way, i.e. when she arrived in the city the slaves were just locked in cages under much crying from their relatives. Nobody bothered telling her about this arrangement before because...why would they? She's just the Queen.
Unsurprisingly Kelsea is shocked and immediately demands that the slaves will be freed, her advisor immediately points out that this would mean war. War with a stronger, more powerful country that has better weapons. He also points out that he fought in the last war with them and how that was an horrible experience. Kelsea still insists that the slaves will be freed.
Now this could be the set-up for great scenes. I mean I do understand her feelings. Despite everything a war is just a hypothetical threat for her while she stands next to the parents who cry while their children are being shipped away. This is not easy...but after her advisor's' initial protests he's immediately fine with her decision without pointing out that freeing the slaves will very likely lead to doom and destruction for basically everybody in her country.