The general Thucydides arrives too late to save Amphipolis from the Spartan Brasidas. He is exiled for twenty years, and uses the time to write his history.
It was also my fate to be an exile from my country for twenty years after my command at Amphipolis
Exile let Thucydides travel freely among Sparta's allies and see the war from the point of view of both sides.
Based on Wikipedia: Thucydides
Using his status as an exile from Athens to travel freely among the Peloponnesian allies, Thucydides was able to view the war from the perspective of both sides.