It wasn't just the Alpha Kingdom that had swung into war mode. News of the movement in the Orc Empire also prompted the Warhammer Kingdom and Moxi Duchy to raise their war readiness levels.
To the orcs, all three lands were Human Kingdoms - worth plundering. Should the North erupt into conflict, all three would invariably be affected.
Even if there were no previous grudges, the repeated harassments from the orcs over time accumulated into deep hatred, turning the orcs into deadly enemies.
The only difference was that the Warhammer Kingdom and the Moxi Duchy had only experienced small-scale invasions, not yet facing the terror of a full-scale orc invasion.
In previous all-out wars, the Alpha Kingdom was at the forefront. It wasn't until the flames of war spread to the homes of the other two nations that the fires reached their peak.