Complex sentence: In particular, the Athenians, who were not protected by the Isthmus, but whose fleet were the key to the security of the Peloponnese, felt hard done by, and demanded an Allied army march north the following year.
Split into simpler sentences: In particular, the Athenians, who were not protected by the Isthmus, but whose fleet were the key to the security of the Peloponnesus, felt hard done by. They demanded an Allied army march north the following year.