3 It began with a vote against Athens at the Spartan assembly
In 432 BC members of the Peloponnesian League, especially those who were troubled by Athens, gathered together at the Spartan assembly. A delegation from Athens was also invited. There was heated debate between Corinthians and Athenians. Corinthians warned Sparta that if it continued to remain passive it will lose its allies and position; while Athenians reminded Sparta of its might. A majority of the Spartan assembly voted against Athens thus effectively declaring war.
4 The first phase of the War is named after Spartan king Archidamus II
The first phase of Peloponnesian War, which is known as the Archidamian War after Sparta’s king Archidamus II, lasted from 431 BC to 421 BC. City-states in the Peloponnesian League, except Corinth, were land-based powers with large nearly unbeatable armies while the Athenian Empire relied on its fleet which was the most powerful in Greece. They maintained their supremacy in their domains and were unable to win any decisive battles.