How did the Geluk school of Tibetan Buddhism begin and why did it eventually become the dominant school of Tibetan Buddhism?

How did the geluk school of tibetan buddhism begin and why did it eventually become the dominant school of tibetan buddhism?
“http://www.treasuryoflives.org/biographies/view/Second-Dalai-Lama-Gendun-Gyatso/11114”
“http://www.treasuryoflives.org/biographies/view/First-Dalai-Lama-Gendun-Drub/10687”

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