Dorne has one of the best troop trees.
Dorne rarely accomplices itself in warfare unless you convince Prince Oberyn Martell otherwise. In many cases, they can actually enter battle and have been spectated to do fairly well. They will join House Targaryen when activated by the questline that brings them into the fray, should you try to have them side in the chaos.
Lore-wise, Dorne is one of the Seven Kingdoms with a sense of independent act towards the overlord Kingship of Joffrey Baratheon, and only by introduction with Princess Myrcella Baratheon should they actually even conclude to bring themselves into Westeros politics. There are many complications and suspected plans that their current monarch Doran Martell is brewing up, but little proof is shown of this in the show, whereas by the books many pieces have already been moved.
Dorne is somewhat isolated, bordered closely by The Reach and The Stormlands. It is also in close proximity to the Stepstones, allowing for easy travel to Essos. Can be recognized by its dark orange colour.
Capital is Sunspear.