Bannermen are expected to provide their military strength to their liege lords. According to George R. R. Martin, "The lords have vassals, the vassals have vassals, and sometimes the vassals of the vassals have vassals, down to the guy who can raise five friends." Poor relationships between liege and bannerman might lead to vassals not providing all or any of their swords, however, or even the bannermen siding with their liege's enemy. Bannermen might hold back their support from a losing cause.
Bannermen can act in their own self-interest, sometimes to the detriment of their liege. Disgraceful actions by a bannerman can bring shame on his liege and homeland. Bannermen can test new lords to judge their worth. Some liege lords pit their bannermen against each other to keep them weak.
Balon Greyjoy rebelled because he did not believe that King Robert I Baratheon had strong support from his bannermen. The other Great Houses did answer the king's call to arms, however, and Robert crushed Greyjoy's Rebellion.
Catelyn: Expect nothing of Walder Frey, and you will never be surprised.
Robb: He's your father's bannerman.Catelyn: Some men take their oaths more seriously than others, Robb.
He had assumed the promise of ten thousand jars was a wild boast, like that of the bannerman who vows to marshal ten thousand swords for his lord and shows up on the day of battle with a hundred and two.—thoughts of Tyrion Lannister
Good men and true will fight for Joffrey, wrongly believing him the true king. A northman might even say the same of Robb Stark. But these lords who flocked to my brother's banners knew him for a usurper. They turned their backs on their rightful king for no better reason than dreams of power and glory, and I have marked them for what they are. Pardoned them, yes. Forgiven. But not forgotten.
Every great lord has unruly bannermen who envy him his place. My father had the Reynes and Tarbecks, the Tyrells have the Florents, Hoster Tully had Walder Frey. Only strength keeps such men in their place. The moment they smell weakness ... during the Age of Heroes, the Boltons used to flay the Starks and wear their skins as cloaks.
A son has a duty to obey his father, true, but a bannerman must obey his overlord.
- A Game of Thrones, Chapter 28, Catelyn V.
- So Spake Martin: Social Structure, Moat Cailin, and More, June 10, 2002
- A Dance with Dragons, Chapter 41, The Turncloak.
- Fire & Blood, Aegon's Conquest.
- So Spake Martin: The Siege of Storm's End, July 18, 1999
- A Game of Thrones, Chapter 53, Bran VI.
- A Storm of Swords, Chapter 66, Tyrion IX.
- A Storm of Swords, Chapter 37, Jaime V.
- A Game of Thrones, Chapter 12, Eddard II.
- The World of Ice & Fire, The Riverlands: House Tully.
- A Clash of Kings, Chapter 25, Tyrion VI.
- A Feast for Crows, Chapter 32, Cersei VII.
- So Spake Martin: Tidbits from the Con (Worldcon, Philadelphia, PA), September 01, 2001
- A Game of Thrones, Chapter 55, Catelyn VIII.
- A Game of Thrones, Chapter 59, Catelyn IX.
- A Clash of Kings, Chapter 20, Tyrion V.
- A Clash of Kings, Chapter 23, Jon III.
- A Clash of Kings, Chapter 42, Davos II.
- A Storm of Swords, Chapter 62, Jaime VII.
- A Feast for Crows, Chapter 33, Jaime V.