The Peloponnesian War began after Sparta attacked Athens by land
Sparta attacked Athens by sea.
Athens attacked Sparta by land
Athens attacked Sparta by sea

Sparta attacked Athens by land.

You must note that the war was started by Sparta, and that Sparta had a better army, while Athens had a better navy. Since Sparta initiated the war, they were usually able to choose whether the battles would mostly be on land or in the sea. Sparta acted on them having a better land force, and invaded through land, which led to Sparta's victory.

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