Why did Stannis join the rebellion?

Question

We know that before the story of ASOIAF begins, Robert's Rebellion has toppled the Targaryen Dynasty and crowned Robert of House Baratheon to sit on the Iron Throne.

Stannis is described (more so in the books than in the tv-series) as having a strong sense of justice. He even killed his younger brother Renly because he felt it was just and right for himself to become king over his younger brother. It also goes to show family loyalty isn't high on his values but more likely, he sees family loyalty as a just and right thing to do, unless there is a more just and right thing.

So my question is Why did Stannis join the rebellion?

Should he not have sided with the Targaryen because they were the rightful rulers of Westeros? Even if he believed what Aerys II Targaryen (the Mad King) did to trigger the Rebellion was unjust, would he not have still supported Prince Rhaegar? Or is it more important to stay loyal to his elder brother?

Answer 1

Ser Davos actually asks Stannis this question directly.

“It is every man’s duty to remain loyal to his rightful king, even if the lord he serves proves false,” Stannis declared in a tone that brooked no argument.

A desperate folly took hold of Davos, a recklessness akin to madness. “As you remained loyal to King Aerys when your brother raised his banners?” he blurted...

“Aerys, If you only knew... that was a hard choosing. My blood or my liege. My brother or my king.” He grimaced. “Have you ever seen the Iron Throne? The barbs along the back, the ribbons of twisted steel, the jagged ends of swords and knives all tangled up and melted? It is not a comfortable seat, ser. Aerys cut himself so often men took to calling him King Scab, and Maegor the Cruel was murdered in that chair. By that chair, to hear some tell it. It is not a seat where a man can rest at ease. Ofttimes I wonder why my brothers wanted it so desperately.”

As you can see, he kind of trails off towards the end. But if we assume that his musing on the difficulty of ruling, Maegor's murder, and Aerys' incompetence wasn't random, I think it gives us our answer.

Stannis chose Robert because he believed, first and foremost, that his duty was to the realm. Aerys insanity and paranoia tore the realm apart by murdering lords on a whim and demanding Ned and Robert's heads. His brother was a strong charismatic young man who commanded the loyalty of all who met him. In Stannis' reckoning, his oaths to Robert and Aerys equaled out, but it was Robert who was best-suited to weather the hardness of the Iron Throne.

Keep in mind, Stannis says several times that he does not even want the Iron Throne. The reason he wages his war is he believes that the Others will soon attack the realm, and he is the only one who can lead the Seven Kingdoms to victory. If Stannis is willing to entertain kinslaying and black magic the good of the realm, is defying a Mad King for the same reason so dishonorable?

Answer 2

I'm not sure if Stannis has even given any explicit reason why he joined the rebellion, but I think it's easy enough to figure out why. At that point in his life, his sense of duty would almost have demanded it, for two reasons:

First, Robert was not only his older brother, but Lord of the House, and the person Stannis was most directly responsible to. It was his brothers place to decide the political course for his House, and Stannis' place to obey him. So, if Robert decided that House Baratheon was going to rebel, Stannis would have been hard pressed to justify not going along with it.

Second, all Stannis really did during the rebellion was defend Storm's End. At that point, he was part of the rebellion whether he wanted to be or not. His duty to his house and the people living in Storm's End would have driven him to defend their home against the Tyrells, regardless of Stannis's personal feelings, because the alternative would have been the death of many innocent people.

As far as his willingness to order Renly's death, keep in mind that by this time, Stannis had seen his life take a number of bad turns since the rebellion. Most importantly, he always felt that Robert cheated him out of Storm's End by giving it to Renly, and leaving Stannis with Dragonstone. Beyond that, his failure to produce a male heir, the problems with his daughter and his wife, and the influencne of Melisandre on him all contributed to him not being in the most rational frame of mind. When Renly chose to claim the crown, that likely pushed Stannis over the edge.

In other words: Stannis' loyalty to his older brother making a claim against what he felt was an unjust king is a far different thing that Stannis' loyalty to his younger brother that he already felt robbed him of his inheritance and was trying to do it again.

Answer 3

I got the impression Stannis didn't have much of a choice in the matter. Robert started the rebellion when the Mad King killed Rickard and Brandon Stark, as Robert went off to war he left Stannis to guard the home-front. Stannis took almost no aggressive action during the war and instead spent most of his time defending himself during the siege of Storms End. Given that he is a prideful man, he would have not surrendered during the siege.