The Early Roman army was deployed by ancient Rome during its Regal Era and into the early Republic around 300 BC, when the so-called "Polybian" or manipular legion was introduced. Until c. 550 BC, there was probably no "national" Roman army, but a series of clan-based war-bands, which only coalesced into a united force in periods of serious external threat. Around 550 BC, during the period conventionally known as the rule of king Servius Tullius, it appears that a universal levy of eligible adult male citizens was instituted. This development apparently coincided with the introduction of heavy armour for most of the infantry.