What happens to common soldiers (not, for example, Jaime Lannister) who are captured during a war in Westeros?
In particular I'm interested in any mention of what happens with regards to this during the War of the Five Kings.
Essentially, it depends on the war, who the winning side is, who the victim is, and who is wielding the sword
A Game of Thrones
Lady Whent yielded up Harrenhal to Lord Tywin.
In the Battle of the Green Fork, Tyrion defeats a knight who yields and is spared.
A Clash Of Kings
Jon snow yields to the Free Folk (to him known as 'Wildlings' at that time), which they accept (once Jon prooves his worth).
But when Ygritte yields to Jon earlier, Qhorin orders Jon to kill her.
In the melee of Ser Loras and Brienne at Renly's camp, Loras yields to her, and obviously is spared - since it's only a melee.
When Arya and co are beset by The Mountain and his men; Lommy yields, but to no avail since he gets killed anyway - this is because he is injured and is not worth the hassle.
A Storm of Swords
Jaime commands Lord Crakehall:
“Tell them the Mad King is dead,” he commanded. “Spare all those who yield and hold them captive,”
-A Song of Ice and Fire: A Storm of Swords, Part One - Steel and Snow, Jaime.
Tywin offers to let anyone who yields [their castle] to be spared:
"I mean to offer generous terms. Any castle that yields to us will be spared,"
-A Song of Ice and Fire: A Storm of Swords, Part Two - Blood and Gold, Tyrion.
Jaime later finds this out:
“Donnel was wounded in the battle and yielded to Ser Elwood Harte. He was ransomed afterward and pledged his fealty to King Joffrey, as did many other captives.”
-A Song of Ice and Fire: A Storm of Swords, Part Two - Blood and Gold, Jaime.
A Feast for Crows
- When Arianne and co are beset upon by Areo Hotah and his crew he offers them to yield, which they do and they are spared (except Arys)
“Yield, my princess,” the captain called, “else we must slay all but the child and yourself, by your father’s word.”
-A Song of Ice and Fire: A Feast for Crows, The Queeenmaker.- Harras Harlaw spares two out of seven people who attacked him, because they yielded:
“The Knight took Grimston by himself. He planted his standard beneath the castle and defied the Grimms to face him. One did, and then another, and another. He slew them all... well, near enough, two yielded."
-A Song of Ice and Fire: A Feast for Crows, The Reaver.- Jaime and his company come across a band of people hanged by the Brotherhood Without Banners:
“We came on some, the day before last,” said Jaime. Addam Marbrand’s scouts had found them, hanging black-faced beneath a crabapple tree. The corpses had been stripped naked, and each man had a crabapple shoved between his teeth. None bore any wounds; plainly, they had yielded.
-A Song of Ice and Fire: A Feast for Crows, Jaime.
A Dance With Dragons
- After the Free-Folks' siege against the wall, they are defeated and put into a stockade.
Later on, once they declare their fealty to Stannis, they are placed into hovels etc. in/around Mole's Town.
Follow the Rules
According to AWOIAF:
Knights are supposed to be brave, courageous, honorable, and true to their word.
According to the rules of Trial by Combat; if an opponent yields, then you're supposed to let them live.
Cash money
Additionally, a "yield" can be a source of revenue, given that the captor can be ransomed, to their family or overlord, for release. This is much more enticing to an attacker than straight-up killing them.
There is an air of expectation that you will be spared if you yield. Though not always followed-through by all.
Asha Greyjoy summarises this the best when she is contemplating her possibility to yield as urged by Lady Glover:
[A Dance With Dragons spoiler]:
... "give me back my children, and no harm need come to you. Nor to your men.”
The last part was a lie, Asha knew. She might be exchanged, perhaps, shipped back to the Iron Islands to her husband’s loving arms. Her cousins would be ransomed too, as would Tris Botley and a few more of her company, those whose kin had coin enough to buy them back. For the rest it would be the axe, the noose, or the Wall. Still, they have the right to choose.
-A Song of Ice and Fire: A Dance With Dragons, The Wayward Bride.
As a knight of the seven, you are supposed to accept an opponent's surrender. If they yield, you are not to kill them in cold blood, you are supposed to take them prisoner. Of course, not everybody who fights in a war is going to be a knight, and even Knights have better things to do than hauling prisoners around, so odds are that even if you surrender, you may still be killed.
It will all come down to who exactly does the capturing, but in general, you are not going to get a very good name if you go around murdering everybody you come across. There is a reason Gregor Clegane is disliked by just about everybody, he doesn't follow the ethics of war.
It seems like swearing fealty is a pretty big "thing" in Westeros. Swear you will never again pick up arms against me or my family and I will let you go. If you're from a rich family, cough up some money as well while you are at it.
