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Sekaisa was a Celtiberian settlement in the middle Ebro valley, identified with modern Segeda or possibly Belmonte de Gracián — the precise location remains debated. The Belli tribe occupying this territory came to direct Roman attention in 154 BC when Segeda began enlarging its walls in violation of treaty terms, triggering the Celtiberian War and forcing Rome to shift its consular year from March to January to mobilize legions faster — a calendrical change that persisted permanently.