game of thrones - Why did Catelyn side with Renly in the War of Five Kings?

In the episode Garden of Bones (season 2, episode 4), Catelyn sides with Renly instead of Stannis even though Ned supported Stannis.

Extract video here.

Stannis: "It is strange to find you beside my brother, Lady Stark. Your husband was a supporter of my claim. Lord Eddard’s integrity cost him his head. And you sit beside this pretender and chastise me."

And then Lady Stark doesn't really answer and changes the subject.

Answer

They knew Renly's host would a be threat to Lannister power in the Riverlands and possibly Casterly Rock.

In A Clash of Kings while Robb is in Riverrun they learn that Ser Stafford Lannister is gather another host at Casterly Rock. Ser Brynden Tully informs Catelyn of this prior to informing Robb

... "Lord Tywin is not the Kingslayer. He will not rush in heedless. He will wait patiently for Ser Stafford to march before he stirs from behind the walls of Harrenhal."

"Unless..." sais Catelyn.

"Yes?" Ser Brynden prompted.

"Unless he must leave Harrenhal," she said, "to face some other threat."

Her Uncle looked at her thoughtfully. "Lord Renly."

"King Renly." If she would ask help from the man, she would need to grant him the style he had claimed for himself.

"Perhaps." The Blackfish smiled a dangerous smile. "He'll want something, though."

"He'll want what kings always want," she said. "Homage."

A Clash of Kings, Chapter 7

It is my thought that they chose Renly to go to first for two reasons:

1. Renly had the larger host;


2. Stannis's whereabouts were not exactly known. He was rumored to be holed up on Dragonstone which would have been harder to reach than Bitterbridge.

However, none of that really mattered when Stannis decided to lay siege to Renly's host in Chapter 22.

Catelyn admits to Stannis that she is not there to take sides between Renly and Stannis but needs the overall help of the Baratheons if Robb is to win his war.

... Stannis studied her face. "And what cause brings you to the 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 hold out the hand of friendship to all."

A Clash of Kings, Chapter 31