Steven, re: the recent talk about feudal military service and the politics thereof… do you think Lord Selwyn was/is trying to thread that needle? Tarth is a large fief that ought to support a levy of men larger than “one” but one is precisely the number of people who ride forth from Tarth. Brienne isn’t at the head of a column, she’s flying solo. That says to me Selwyn is hedging; if Renly wins his daughter was in his service, if the Lannisters win he can play the “I didn’t send levies” card.

I’m pretty sure that’s exactly what he did. 

We learn in the Prologue to ACOK that only half the Stormlords actually sent men to Renly, while the other half “sit behind their walls waiting to see how the wind rises and who is likely triumph.” And Selwyn Tarth is explicitly listed as belonging to that faction, as Davos reports:

“I broke bread with Gulian Swann and old Penrose, and the Tarths consented to a midnight meeting in a grove.”

So yeah, I think Selwyn held back his levies but let his idealistic daughter pursue her dreams.