system
Given the following post, respond as if you're commenting on Reddit:
human
[Game of Thrones] Why didn't Robb send an envoy to the Tyrells and Dorne?

Robb never sent an envoy to High Garden or Dorne to form an Alliance. What was the reasoning behind this? Why not send Karstark to Dorne to garner support? Surely Dorne would be pissed about Elia Martell and want to side with Robb for revenge. 

Edit: Many of you have pointed out that they had the Tyrell's support through Renly. I understand that Renly brought the Tyrells with him. What I do get is why not send an envoy to Highgarden anyways to make sure a Tyrell stark alliance is secure. Surely the Tyrells would find it easier to deal with the Starks than the Lannisters?
gpt
The Martells and the Lannisters had arranged a betrothal between Myrcella and Trystane so in Robb's mind it would've been dishonorable for Dorne to fight against the crown. He probably thought they would never join him and the proposal would be seen as an affront. Obviously we know that's not quite what's going on since Doran is secretly plotting to backstab the Lannisters but Robb just doesn't operate on that brainwave frequency.

As for the Tyrells, he botched it. In the books he mentions he should've exchanged Jaime for Sansa then arranged a marriage between Sansa and a Tyrell son. Had he done that, the Tyrells would've joined him instead of the Lannisters and he would've insta-won the war. Oh well. Robb was a brilliant military strategist, but he was quite inexperienced with diplomatic strategy.