Malachy, in the first year of his reign, having received the hostages of the Danes of Dublin, having liberated the Irish prisoners and secured the unity of his own territory, had his attention drawn, naturally enough, towards Brian's movements. Whether Brian had refused him homage, or that his revival of the old claim to the half-kingdom was his offence, or from whatever immediate cause, Malachy marched southwards, enforcing homage as he went. Entering Thomond he plundered the Dalcassians, and marching to the mound at Adair, where, under an old oak, the kings of Thomond had long been inaugurated, he caused it to be "dug from the earth with its roots," and cut into pieces. This act of Malachy's certainly bespeaks an embittered and aggressive spirit, and the provocation must, indeed, have been grievous to palliate so barbarous an action. But we are not informed what the provocation was. At the time Brian was in Ossory enforcing his tribute; the next year we find him seizing the person of Gilla-Patrick, Lord of Ossory, and soon after he burst into Meath, avenging with fire and sword the wanton destruction of his ancestral oak. Thus were these two powerful Princes openly embroiled with each other. We have no desire to dwell on all the details of their struggle, which continued for fully twenty years. About the year 987, Brian was practically king of half Ireland, and having the power, (though not the title,) he did not suffer any part of it to lie waste. His activity was incapable of exhaustion; in Ossory, in Leinster, in Connaught, his voice and his arm were felt everywhere. But a divided authority was of necessity so favourable to invasion, that the Danish power began to loom up to its old proportions. Sitrick, "with the silken beard," one of the ablest of Danish leaders, was then at Dublin, and his occasional incursions were so formidable, that they produced (what probably nothing else could have done) an alliance between Brian and Malachy, which lasted for three years, and was productive of the best consequences. Thus, in 997, they imposed their yoke on Dublin, taking "hostages and jewels" from the foreigners. Reinforcements arriving from the North, the indomitable Danes proceeded to plunder Leinster, but were routed by Brian and Malachy at Glen-Mama, in Wicklow, with the loss of 6,000 men and all their chief captains. Immediately after this victory the two kings, according to the Annals, "entered into Dublin, and the fort thereof, and there remained seven nights, and at their departure took all the gold, silver, hangings, and other precious things that were there with them, burnt the town, broke down the fort, and banished Sitrick from thence" (A.D. 999).