Casca The Prussian by Tony Roberts

Hunting down a man described by the woman who had hired him as her late husband's killer, Casca becomes hunted in turn by French soldiers. To escape, the Eternal Mercenary joins the Prussian army of King Frederick as war breaks out in Europe.

Together with a group of other new recruits, he forms part of an elite guard unit sent to fight against the Austrians and French in a war that looks like it will have no easy outcome. Fighting in a series of tough battles, he learns that the Austrians are no pushovers, and to add to the Prussians' problems, Russia joins in the war against them.

Casca's battles are not just limited to the battleground against the three enemy nations; he also has to deal with a hostile member of his own unit who takes exception to the Eternal Mercenary's sense of justice, and has to face yet another 'death' and painful rebirth before he can exact revenge on the man who wronged him.