The Emperor's Men 1: Arrival

Shortly before the First World War, the German light cruiser Saarbrücken leaves the port of Wilhelmshaven to make its last big journey before being decommissioned. But near Portugal the old ship encounters a mysterious phenomenon, and the crew unexpectedly finds itself in the Mediterranean, 1500 years in the past, at a historical moment: It's the year 378, the beginning of the end of the Western Roman Empire, the start of Völkerwanderung.the crew of the Saarbrücken decides that to survive they must make friends of the Romans.
They had been the Kaiser's men in the 20th century, and now, in the past, another emperor might need their services.