What's fucked is that the warrior cats clans have little or no interdependence to speak of, which means that diplomacy is almost completely toothless when there's nothing the clans can offer or withhold from each other.
In real life if, say, one country does something shitty to another country, there are at least some things that can be done before escalating to military action, ways to mete out consequences without going to war: lawsuits, trade sanctions, travel restrictions, and so on. But the clans don't trade with each other, they rarely collaborate outside of extreme circumstances, and borders are permanently closed by default, so the only real way to retaliate at all is to escalate to violence. And even in quote-unquote peacetime, except in the aforementioned extreme circumstances, no clan ever really benefits from the existence of the other clans; not only are there no systems in place for sharing resources or skills, most characters in-universe would react with suspicion or confusion to the idea that they even should. The ideal situation in the warrior cats world is one where the clans don't interact at all outside of monthly Gatherings. This cat society is built in such a way that peace is synonymous with isolation. Travel between clans is impossible because every adult member of each clan is an active member of that clan's military unless they're on maternity leave or retired, which makes every crossing of a border an automatic act of aggression. Warrior Cats is the story of four military dictatorships shakily coexisting for months at a time until something inevitably raises tension and hostilities again, and both the characters and the narrative act outraged over idea that maybe they should be less of a military dictatorship.