The Tholos at Delphi constructed around 420 BCE marks the first emergence of a round temple form.

The newly designed Corinthian capitals make their initial appearance on temple exteriors around the same time.

The Nike of Paeonius is erected outside the Temple of Zeus at Olympia in about 420 BCE.

According to the inscription on the base, it was dedicated by the people of Messenia and Naupactus after a victory in an unnamed conflict, possibly the battle of Sphacteria.

Paeonius also won the commission to decorate the acroteria of the Temple of Zeus at Olympia (about 430 - 420 BCE), Pausanias was probably wrong in claiming that he worked on the sculptures for the pediments of the temple.

The Nike of Paeonius will be featured as part of the design of the medals of the 2004 Summer Olympics in Athens and is today exhibited at the Archaeological Museum of Olympia.

The city of Elis, again allied with Sparta during this phase of the Peloponnesian War, in 420 defects to the side of Athens, as does …

…Argos and …

…Mantineia.

Ictinus’s highly original Temple of Apollo Epicurius at Bassae in Arcadia, executed around 420, incorporates engaged interior columns and the first known use in architecture of the Corinthian order.

Ictinus transforms the interior space into an area for public assembly—a development that mandates a return to the long cella.

Agis II, king of Sparta from about 427 BCE, invades the territory of Athens' ally Argos in 418 BCE while the Peace of Callias (421-415 BCE) is still in effect, but inexplicably makes a truce and withdraws after cutting the Argive army off from its city.

He escapes heavy penalties for his failure to press his advantage by promising more successful enterprises.

He receives his chance a few weeks later, when …

…the anti-Spartan and aggressive Alcibiades, who in 420 BCE had been named general for the first time, opposes the Nicias, who had negotiated the peace, forces a break with Sparta, and steers Athens into …

…the anti-Spartan alliance formed by the Peloponnesian city-states Argos, Elis, and Mantineia, all formerly in the Spartan camp.

The Spartan forces of King Agis crushingly defeat the alliance of Athens, Argos, Elis, and Mantineia at the Battle of Mantineia (418), in which the Athenian commander, Laches, is killed. (A dialogue of Plato's, on bravery, is known by his name.)