The War of the Twelve Kingdoms or 12 Kingdoms War was an event of paramount importance which took place 300 years prior to the events of the Mai-Otome anime.
The war was the nadir of civilisation on the planet Earl and utilised advanced technology, much of it of Earthen manufacture, including aircraft, as well as many androids of a type later collected by Schwartz for the purposes of reprising this type of conflict. The war also involved the use of the Harmonium in Windbloom, which resulted in massive destruction, the disappearance of Black Valley and the 300-year "curse" on the Aswad people. The war was eventually ended by the intervention of Fumi Himeno, the first Otome and Founder of the Garderobe Academy.
The 12 kingdoms, as well as the 300 year time lapse between the war and the present, suggest links with the HiME Carnival - the last HiME Carnival conducted without interference from the Searrs Foundation occurred 300 years prior to the events of the Mai-HiME anime in a similar vein to the 12 Kingdoms War in relation to Mai-Otome.