In the twelfth century Ireland was full of Englishmen who had been kidnapped and sold into slavery in spite of royal prohibitions and the spiritual menaces of the English Church.

The slave-trade afforded a legitimate pretext for war, had a pretext been needed by the ambition of Henry the Second; and within a few months of that king's coronation John of Salisbury was despatched to obtain the Papal sanction for an invasion of the island.

The enterprise, as it was laid before Pope Hadrian IV., took the colour of a crusade.

The isolation of Ireland from the general body of Christendom, the absence of learning and civilization, the scandalous vices of its people, were alleged as the grounds of Henry's action.

It was the general belief of the time that all islands fell under the jurisdiction of the Papal See, and it was as a possession of the Roman Church that Henry sought Hadrian's permission to enter Ireland.