With the death of Quentyn after his rejection by Danaerys in Mereen and the seemingly-instant affection for Aegon "Young Griff" Targaryen, who most people believe to be a fake, there are widepsread predictions by some of the most brilliant thinkers online that the Martells are going down. The thought is that Doran is so attached to vengeance, just like the Sand Snakes, that he will hitch his horse (via marriage) to any Targaryen able to help his family get back into power.
There is every reason to expect this behavior of Doran. We learn of a secret marriage pact negotiated between Oberyn Martell and Willem Dary for the marriage of Viserys and Arianne Martell. We see how Tyrion arranged the betrothal of his niece Myrcella and Doran's son Trystane. We read how Arianne is sent by Doran to check out young Aegon after Jon Connington sent a letter claiming that Elia's son is actually alive and asking for Dorne's help.
But there's more to the Martells than we have been able to see.
I want to back up and think fundamentally what is driving Dorne. Is it emotion -- a thirst for vengeance above all? Or does Doran act like a true statesman, seeking to forward the interests of Dorne, as the international relations theory called realism would suggest? In the modern era, we learn how even "mad" dictators like the North Korean Kim family still act like rational actors when we look at them from the perspective of geopolitics. Why wouldn't we apply the same lens to Dorne?
I want to grant that Doran is a fictional character, and that George R.R. Martin may not have been thinking as I suspect he did when he wrote the books. Doran could be someone beset with the thirst for vengeance. Still, I'm reminded of the question Martin asked about what Aragon's tax policy was. In other words, he acknowledges that states' decisions are based more than the emotional impulses of their rulers, but generally on some rational basis.
So what would Dorne decide to do if it was rational?
Let's think about the history of Dorne. The Martells' words "Unbowed, Unbent, Unbroken" mean that they were never conquered, they never surrendered, and they never were smashed in battle. Aside from the history in Fire and Blood to the contrary, these words are mostly true. And those exceptions are important in framing what Dorne wants now. Aegon the Conqueror was never able to overcome it, and at Hellholt, his sister Rhaenes Targaryen and her dragon Meraxes fell from the sky after a scorpion bolt hit the beast in the eye. Armies from the Reach met their demise in marching across the red sands of Dorne. It was only after Prince Maron Martell was married to Danaerys Targaryen in 187 AC that a Dornish prince "bent the knee."
Geopolitically speaking, for much of the post-Conquest history, Dorne has acted much as Russia did within European politics. Relying on a large swath of land between other kingdoms and inhospitable weather, Dorne, like Russia, made itself the graveyard of conquerors. In fact, there are so many similarities between Dorne and Russia in their response to invasion (i.e., abandonment of cities and fortresses, harrying attacks on invading armies) that it's possible GRRM partly based Dorne on Imperial Russia. Russia's response to Napoleon's invasion in 1812 provides an amazing real-world parallel to these strategies employed by Dorne during Aegon's conquest. [As an aside, the "Mossovy" in Essos is remarkably similar to the medieval antecedent of the Russian state: Muscovy -- Moscow.]
With a degree of security unique in Westeros, Dorne remains a kingdom apart. The Martells are difficult to remove from power, and Dornish land is nigh-impossible for foreign forces to occupy. Considering this position of geopolitical strength, what would cause any Dornish leader to sacrifice this security by drawing it into a war for the Iron Throne?
Most of the fandom and the (brilliant) online commentariat seem to think that Doran Martell is locked into the same approach to maintaining power that Prince Maron Martell employed about 120 years ago: marrying a Targaryen. However, that's not the only path forward for Dorne.
It's important to keep in mind that the secret wedding pact signed by Oberyn Martell with Willem Darry took place after Targaryen rule had ended in Westeros. The secret wedding pact was far from a trump card in the game of thrones -- more of a way to diversify Dorne's investments. Note that it was only after "our" Danaerys had conquered Meereen that Doran sent Quentyn to court her. It was only after receiving a letter from Jon Connington that Doran sent Arianne to investigate Aegon/Young Griff in the Stormlands.
At the end of Robert's Rebellion, how likely was it that Viserys would be able to survive long enough to make a claim on the Iron Throne? It seems like an extremely risky bet for Doran to put all his political chips on the marriage pact. While later, Doran tries to use the pact to join his family with Danaerys, at the end of Robert's Rebellion, that wasn't known to him.
We do know from Tywin Lannister that at the end of Robert's Rebellion, Prince Oberyn Martell "tried to raise Dorne for Viserys." Yet somehow, that uprising never happened... likely because Doran prevented it.
So, if Doran is a master player of the game of thrones, what other bets might he have placed at the end of Robert's Rebellion?
I've got one theory... please stay tuned!
