Final answer:
Internal conflict within Athens invited Sparta's external threat, leading to the Peloponnesian War. The war demonstrated how internal strife can undermine a state's stability and invite external intervention, eventually resulting in the downfall of Athens.
Step-by-step explanation:
A conflict within a city-state like Athens can invite threats from external forces such as Sparta due to perceived vulnerabilities. The Peloponnesian War between Athens and Sparta began in part because of internal strife among their allied poleis, leading to a perception of opportunity for intervention by external powers. Open war broke out when Athens attempted to influence the political decisions of Spartan allies, which, combined with Athens's imperial ambitions manifested through the Delian League, and Sparta's dominance in the Peloponnesian League, escalated tensions leading to a full-scale war.
Athens's involvement in assisting Corcyra against Corinth, a member of the Peloponnesian League, further provoked the Spartans. Furthermore, during the war, both sides looked for allies beyond Greece, with Sparta even allying with its former enemy, Persia. These external alliances and the support gathered from outside of Greece were vital in the eventual Spartan victory and the downfall of Athens, demonstrating the consequences of internal conflicts spilling over and becoming larger regional battles.
Therefore, internal conflict weakens a state's stability, making it more susceptible to external threats who may see the discord as an opportunity to undermine or dominate the weakened entity. Athens and Sparta's struggle for control and influence provides a stark example of this phenomenon, inviting external interference and eventual decimation of Athenian power and autonomy.