What is the theme in A Storm of Swords by George R.R. Martin?

One imortant theme in the novel is that of betrayal.

As the struggle for power continues, betrayal becomes commonplace. Mercenaries switch sides to whoever is paying the most. Families align themselves with one side or another, but will change alliances if the need arises. Betrayal is a common theme of war, as one side tries to encroach on the other. If one side is honorable, it only makes it easier.

The largest incident of betrayal in the novel is the Red Wedding. Robb Stark is a noble man, and he marries Jeyne Westerling because he took her virginity, and to not do so would be to dishonor her. His nobility causes him to offend the Freys, as he promised to marry one of their daughters. The Freys take this as a betrayal, and respond with a bigger one. They ally with the Lannisters to lure Robb Stark and his men to the Twins, obstensibly for Edmure Stark's wedding to Roslin Frey. The Starks have guest rights, which according to the honor code of the Seven Kingdoms, means they will not be harmed. However, the Freys still attack and kill the Starks. They betray not only the Starks, but also the fidelity of the Kingdom by violating the right of guests.

Within the betrayal by the Freys, Rob is betrayed by Roose Bolton, a supposed ally. His betrayal is foreshadowed when Arya Stark escapes from his camp, for if Bolton was an ally of the Starks, there would be no need for him to hold Arya and not return her to her mother. Also, when Jaime Lannister is brought to Roose, he lets him go back to King's Landing. Jaime is not aware of the betrayal plot, but he tells Roose to give his regards to Robb. At the Red Wedding, an armored knight stabs Robb and says these words, indicating Roose is the one who actually kills Robb. In effect, Roose also betrays Jaime Lannister, as saying Jaime sends his regards indicates Jaime masterminded the plot. While Jaime might have liked Robb dead, this is not true and may have repercussions.

Betrayal is also evident among the Night's Watch. At the beginning of the novel, Chett plans to betray his comrades and deserts, stopped only by the Other's attack. While Jon Snow is acquitted of betraying the Night's Watch, he does betray the wildings. When he is captured, he gains their trust and becomes one of them, or so they think. When the wildings plan to move on Castle Black, Jon cannot be disloyal to the Night's Watch and warns them, but he betrays the wildings in turn. Even though he does not fully consider himself a wilding, Jon leads Mance Rayder to believe he is committed to their cause, and betrays their trust. He honors the code of the Night's Watch, but his honor is what leads him to betrayal, much as in the case of Robb Stark.