From the ninth century, the taking of slaves was an integral part of Viking warfare. Though never the prime motive for raiding, it was a means of indicating defiance and was followed up by the extraction of ransom and tribute. Slave-trading with Scandinavia and Iceland developed slowly. In the eleventh century, when the Irish internal struggle for over-kingship escalated, the taking of slaves became a widespread phenomenon. Warring Irish kings sold prisoners of war in the Dublin slave-market and Dublin experienced a growing slave-trade with western Europe. In the second half of the eleventh century, there seems to have developed a specific Irish-Sea slave-market, but in the twelfth century Norman legislation against the slave-trade seems to have been effective and Dublin’s control of the Irish Sea was broken.