Which Greek city-state was known for its strict military discipline and rivalry with Athens?

Sparta was renowned for its strict military discipline and its unique social structure that prioritized military training and prowess above all else. The city-state developed an education system known as the Agoge, which was designed to prepare male citizens for a life of military service from a very young age. This emphasis on discipline, physical strength, and combat readiness created a society that was both formidable in warfare and deeply connected to martial values.

Sparta's rivalry with Athens stemmed largely from their differing lifestyles and political systems. While Athens was known for its democratic principles, cultural achievements, and emphasis on arts and philosophy, Sparta focused on a powerful and austere military culture. This contrast ultimately contributed to significant conflicts, including the Peloponnesian War, where the rivalry between these two city-states reached its peak. Thus, choosing Sparta as the answer clearly identifies it as the city-state at the center of military discipline and contention with Athens.