The Custom of the Army (Lord John Grey #2.75) by Diana Gabaldon

London, 1759. After a high society electric-eel party leads to a duel that ends badly, Lord John Grey feels the need to lie low for a while.

Conveniently, before starting his new commission in His Majesty’s army, Lord John receives an urgent summons. An old friend from the military, Charlie Carruthers, is facing court-martial in Canada, and has called upon Lord John to serve as his character witness. Grey voyages to the New World—a land rife with savages (many of them on his own side) and cleft by war—where he soon finds that he must defend not only his friend’s life but his own.

Note: Previously included in the anthology Warriors.