The Silver Branch

Blurb

The Silver Branch is a historical adventure novel for children written by Rosemary Sutcliff and published in 1957, with illustrations by Charles Keeping. Set in Britain in the last decade of the 3rd century, it is the story of Justin and Flavius, two cousins in the Roman legions who find themselves in the intrigue and battle surrounding the struggles between Carausius, a self-proclaimed emperor in Britain, Allectus, Carausius' treasurer, and Constantius, emperor in Rome.
The silver branch of the title is an article in the possession of a friend of the characters. Subtle allusions to it recur in other novels in the Eagle of the Ninth series, and it presumably refers to the otherworldly instrument mentioned in the medieval Irish narrative The Voyage of Bran. The Silver Branch was also a symbol of authority and temporary "pass card" used by the Celts, consisting of a sprig of Mistletoe or an apple branch.
The Silver Branch is #2 in the Roman Britain Series, following The Eagle of the Ninth and preceding Frontier Wolf, The Lantern Bearers, and Sword at Sunset.