420s

The 420s refers to the decade spanning the years 420 through 429, in a given calendar era. It is used when dating historical events or other time-specific topics, and the exact meaning depends on whether one is discussing the BCE/BC or CE/AD era.

In the 420s CE, late antiquity was marked by political and military reshaping across the Roman world. The Western Roman Empire continued to contend with external pressures from various groups while internal authority remained fragile; Valentinian III became emperor in 425, reinforcing the continuity of imperial rule in the West. In the Eastern Roman Empire, leadership under Theodosius II continued frontier management and religious policy as the empire faced broader structural changes occurring during this period.

In the 420s BCE, the decade falls within the later stages of the Peloponnesian War in ancient Greece. The decade witnessed continued military campaigns, shifting alliances, and political realignments among city-states such as Athens and Sparta, with the broader war dynamically shaping the Greek world of the time. The Peace of Nicias, concluded in 421 BCE, is one of the notable milestones associated with this period, though subsequent hostilities soon reemerged.

Beyond historical practice, the term 420s is not widely used to denote a modern concept; the number 420 is more commonly associated with cultural references tied to cannabis culture, but the plural form 420s is primarily a historical framing device in scholarly discussions of time periods.

In summary, 420s denotes the decade of 420–429 in a chosen era, with distinct historical contexts in CE/AD and BCE/BC that reflect the broader political and military dynamics of late antiquity and classical Greece.