O Skooba acrescentou uma boa resposta, mas há uma coisa que ele não mencionou do ponto de vista do livro. Desde que você adicionou a tag de livros, eu suponho que devo mencioná-la.
Por que escolher o Renly?
Em livros, os nortistas não sabiam (naquela época) toda a história sobre Joffrey ser um bastardo. Para eles, Stannis e Renly eram pretendentes, para usurpar o trono do sobrinho.
Para eles, as escolhas foram:
- Rei Joffrey, que assassinou Eddard.
- Príncipe Tommen, outro nascido de Cersei Lannister, portanto não menos Lannister, como apontado pelos Lordes Riverlander quando Robb disse que Tommen é o herdeiro de Joffrey.
- Lorde Stannis, o senhor azedo e severo que tinha um número muito pequeno de tropas ou recursos para oferecer, mas tinha uma reivindicação melhor do que seu irmão.
- Lorde Renly, o irmão alegre, bem vestido e amado do falecido rei, que tinha um exército enorme, mas não uma afirmação muito boa.
Eles estavam considerando suas opções no AGOT Catelyn IX :
Lord Jonos Bracken rose to insist they ought pledge their fealty to King Renly, and move south to join their might to his.
“Renly is not the king,” Robb said. It was the first time her son had spoken. Like his father, he knew how to listen.
“You cannot mean to hold to Joffrey, my lord,” Galbart Glover said.
“He put your father to death.”
“That makes him evil,” Robb replied. “I do not know that it makes
Renly king. Joffrey is still Robert’s eldest trueborn son, so the
throne is rightfully his by all the laws of the realm. Were he to die,
and I mean to see that he does, he has a younger brother. Tommen is
next in line after Joffrey.”
"Tommen is no less a Lannister," Ser Marq Piper snapped.
"As you say," said Robb, troubled. "Yet if neither one is king, still,
how could it be Lord Renly? He's Robert's younger brother. Bran can't be Lord of Winterfell before me, and Renly can't be king before
Lord Stannis."
"Renly is crowned," said Marq Piper. "Highgarden and Storm's End
support his claim, and the Dornishmen will not be laggardly. If
Winterfell and Riverrun add their strength to his, he will have five
of the seven great houses behind him. Six, if the Arryns bestir
themselves! Six against the Rock! My lords, within the year, we will
have all their heads on pikes, the queen and the boy king, Lord Tywin,
the Imp, the Kingslayer, Ser Kevan, all of them! That is what we shall
win if we join with King Renly. What does Lord Stannis have against
that, that we should cast it all aside?"
“The right,” said Robb stubbornly. Catelyn thought he sounded
eerily like his father as he said it.
Então, até esse ponto, Robb estava predisposto a Lorde Stannis em vez de Renly. Foi a Proclamação de Great Jon Umber, que mudou tudo. Citando o mesmo capítulo:
“MY LORDS!” he shouted, his voice booming off the rafters. “Here is
what I say to these two kings!” He spat. “Renly Baratheon is nothing
to me, nor Stannis neither. Why should they rule over me and mine,
from some flowery seat in Highgarden or Dorne? What do they know of
the Wall or the wolfswood or the barrows of the First Men? Even their
gods are wrong. The Others take the Lannisters too, I’ve had a
bellyful of them.” He reached back over his shoulder and drew his
immense two-handed greatsword. “Why shouldn’t we rule ourselves again?
It was the dragons we married, and the dragons are all dead!” He
pointed at Robb with the blade. “There sits the only king I mean to
bow my knee to, m’lords,” he thundered. “The King in the North!”
Agora os nórdicos não estavam procurando por um Overlord, que tinha que ter uma boa reivindicação ao trono. Agora eles eram independentes, negociando como iguais e tinham que encontrar um aliado que pudesse ajudá-los. A escolha lógica era o rei Renly, que tinha Stormlands e o Reach atrás dele.
De quem foi a aliança com Renly?
Skooba já apontou corretamente que era Catelyn quem sabia o que fazer quando as notícias de outro Exército Lannister chegaram a eles.
Stannis sabia sobre a lealdade de Eddard?
Para não mencionar, a citação que você mencionou é uma única coisa show. Nos livros, Stannis nunca soube que Lord Eddard era a favor de sua reivindicação.
Como já foi mencionado pelo Skooba, foi assim:
Stannis studied her face. “And what cause brings you to this field,
my lady? Has House Stark cast its lot with my brother, is that the way
of it?”
This one will never bend, she thought, yet she must try nonetheless.
Too much was at stake. “My son reigns as King in the North, by the
will of our lords and people. He bends the knee to no man, but holds
out the hand of friendship to all.”
Resposta da Catelyn
Catelyn não ficou quieta. Ela respondeu a Stannis com franqueza e mostrou que ela e seu grupo não sabiam da verdade obscura por trás da filiação de Joffrey:
Stannis frowned at her. “You presume too much, Lady Stark. I am the
rightful king, and your son no less a traitor than my brother here.
His day will come as well.”
The naked threat fanned her fury. “You are very free to name others
traitor and usurper, my lord, yet how are you any different? You say
you alone are the rightful king, yet it seems to me that Robert had
two sons. By all the laws of the Seven Kingdoms, Prince Joffrey is
his rightful heir, and Tommen after him... and we are all traitors,
however good our reasons.”
ACOK- Catelyn III