At Vox Popoli, Vox Day quotes from an account during the Roman Empire of Visigoths fleeing Huns. The onslaught eventually pushed the Visigoths against the Danube River, which formed the border of the Roman Empire. The Visigoths begged the Empire to let them cross the river into Roman territory. As described by an observer of that period:
All the multitude that had escaped from the murderous savagery of the Huns— no less than 200,000 fighting men, besides women and old men and children— were there on the river bank… promising that they would ever faithfully adhere to the imperial alliance if only the boon was granted them.
Emperor Valens decided to be charitable and— he thought— prudent. He told the Goths they could cross into Roman territory if they would surrender their weapons and provide hostages to the Romans. Soon after they had crossed the river they started refusing to surrender their weapons. Hungry Goths attacked a marketplace and when Roman soldiers tried to repel them, they killed the soldiers.
As to the hostages,
The Goths drew their swords and cut their way out of the palace. Then riding to the nearest camp of his followers, Fritigern told his tale, and bade them take up arms against Rome. There followed a year of desperate fighting all along the Danube…
Eventually things came to a head in a battle against the Goths, led by the Emperor himself.
The Romans were slaughtered.
More than forty thousand of them were killed, including the Emperor who had so mercifully allowed the Visigoth refugees to enter his land.
It gets worse. As Day points out,
38 years after the Goths crossed the Danube, Alaric the Goth sacked Rome itself.
And that, my dear bleeding heart moralists, is why you always sink the damn ships.