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In page Seibal:

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The earliest mention of Seibal in hieroglyphic inscriptions is on Stela 15 at Dos Pilas, dated to 13 October 721, and refers to Seibal becoming involved in the long-running war between the great cities of Tikal and Calakmul and their respective allies and vassals. Dos Pilas was a newly founded city in the Petexbatún region, established by Tikal to exert its control over the Pasión River, a strategy that backfired when Calakmul seized control of the fledgling kingdom.[6]