The Warring Ages (戦乱期 Senranki) was the period of time between roughly 1000 years before present (BP) and several hundred years BP in the Nanoha universe. The exact dating is yet to be clarified by canon sources.
During these centuries, Belka rapidly expanded their presence in the multiverse, using the advanced magical technology leaked from Al-Hazard and mass-based weaponry. As the name of the time period implies, their main method of expansion was armed conquest, leading to the development of the combat-oriented Ancient Belkan magic system. It was also during this time that the Saint Kings, Belkan supreme rulers, began magically strengthening their bodies.
The Warring Ages ended when the Belkan homeworld became uninhabitable due to centuries of pollution (possibly caused by unrestricted use of mass-based weapons) and the Ancient Belkan empire collapsed. An alternative theory states that Belka, along with many nearby worlds, was destroyed in a large-scale dimensional dislocation that happened in the year 462 of the old Mid-Childan calendar.
The collapse of the Belkan empire was immediately followed by the Saint King Unification War.
- History of Belka at the Japanese Nanoha wiki