Do you think there is something unwritten about Otto Hightower’s ability to form coalitions? I mean he was able to pull the Free cities into the Dance so it would appear that he had the ability to garner beneficial relationships. Otherwise the fact that the Greens were able to keep their coalition together in the face of disaster after disaster in the Dance like that obviously stretches one’s ability to suspend disbelief. You would think it was Rome and its allies up until Hannible at Cannae.

I suppose. On the other hand, one does have to wonder how it is that Otto Hightower, despite being Hand of the King and with a Hightower regent in Highgarden raising Lord Lyonel, couldn’t rally the Reach around his cause, to the point where Oldtown’s forces couldn’t even make it out of the Reach without getting brutalized at the Battle of the Honeywine. 

I think the answer is that the Dance is not one of GRRM’s best-plotted wars/historical events, and relies rather heavily on jobbing to get the greens to lose to the blacks.