Athens transformed the Delian League (anti-Persian alliance) into an empire, forcibly incorporating or subduing numerous city-states. Key conquests:
Island rival, naval threat. Athens besieged and occupied it, expelled Aeginetans, installed cleruchy (settler colony). Critical for Saronic Gulf control.
Strategic isthmus city. Athens seized it, built "Long Walls" to port Nisaea, securing land route to Corinthian Gulf. Later lost (446 BCE) in Thirty Years' Peace.
Including Thebes and Plataea. Athens imposed democratic regimes after Battle of Oenophyta. Lost decisively at Battle of Coronea (447 BCE) — Boeotia regained independence.
Large island off Attica. Revolted 446 BCE; Athens crushed revolt, confiscated land (Hippobotai), installed cleruchies. Remained tributary.
Cycladic island. Attempted secession from Delian League. Athens besieged, forced surrender, stripped fleet, imposed tribute. First major league member reduced to subject status.
Neutral Cycladic island. Athens demanded submission; Melians refused. Siege, surrender, execution of adult males, enslavement of women/children, Athenian cleruchy. Thucydides' Melian Dialogue epitomizes realpolitik.
These actions fueled resentment, driving the Peloponnesian War (431–404 BCE) and Athens' eventual defeat. The "Athenian Empire" was a hegemony of coerced tribute and strategic garrisons, not territorial annexation in the modern sense.