Sparta invades Athens because Sparta feels that Athens is abusing its power as head of the Delian League. At the beginning of the war, there is a plague in Athens that kills 1/3 of the population, including their leader Perocles. This puts Athens at a huge disadvantage. The fighting continues with Sparta winning until 421 B.C., when a temporary truce is called so they can regroup. Fighting resumes, and in 415 B.C., Athens changes their strategy of waiting for Sparta to come to them and invades Syracuse, one of Sparta's closest allies. Sparta is able to save Syracuse from being invaded, and crushes the Athenian army. After another 9 years of ongoing warfare with Sparta still winning, Athens finally surrenders and loses their power and wealth.