The Leopard Sword — Michael Cadnum

Ragged and exhausted from fighting in the Crusades, Hubert, a knight's squire, and his fellow crusaders board a ship bound for England. But the return home is neither quick nor easy. Hubert must use his sword against ships full of attacking infidels, and he is nearly slain at the hand of a vengeful knight. A shipwreck strands them in Rome, where the beautiful and brave Lady Galena nearly tempts him to stay. Instead, loyalty to his master prevails and they finally reach their beloved land-only to find that one of their group faces execution when he steps ashore. If Hubert is to save his friend's life, he must first save his own.