Arcadia maintains a network of alliances known as the Green Circle with various other nations of the Discworld (And a few nations off in Shadow). While Arcadia is the dominant partner, it has yet to progress to the level of supreme dominance which Amber has with regard to the Golden Circle.
Members of the Green Circle accord full trading rights to each other's citizens, and are expected to come to each other's aid in times of war. Each nation has full internal autonomy, and in general stay out of each other's internal politics, though Arcadia has been known to meddle (for their own good, of course).
Notable Green Circle Kingdoms: