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In page Brian Boru:

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Throughout his campaigns in the province, it was once again Brian's coordinated use of forces on land and at sea that allowed him to triumph; while the rulers of Ulster could bring the advance of Brian's army to a halt[citation needed], they could not prevent his fleet from attacking the shores of their kingdoms. Once Brian entered the province of Ulster, he systematically defeated each of the regional rulers who defied him, forcing them to recognise him as their overlord.