The growth of the Athenian Empire and the power accruing to Athens aroused the fears of Sparta and other mainland states, especially Corinth, whose trade interests seemed to be threatened by Athens's control of the sea and so many of the islands and ports of the Aegean Sea. The Greek world split into two blocks of states, Athens and her Empire on one side, Sparta and her allies on the other. Both sides expected war, and it broke out in 431 over incidents in Corcyra and Potidaea (in northern Greece). Known as the Peloponnesian War, the conflict lasted (off and on) until 404, when Athens was defeated.
Thucydides, an Athenian aristocrat, was probably in his late twenties at the time the War began; he realized its importance from the start and began to plan to write its history. In 424 he was elected one of the Athenian generals, and for failing to prevent the loss of an important city to the Spartans was exiled from Athens. He spent the rest of the War collecting evidence and talking with participants in the various actions. Herodotus, writing a few decades earlier than Thucydides, recorded almost all he heard, whether he believed it himself or not. Thucydides stands at the other pole; he gathers all available evidence, decides what he thinks is the truth, then shapes his presentation to emphasize that truth. We see everything through his eyes, and his views on the forces which shape human events emerge on every page.
Thucydides begins his history by explaining why he thinks that this War is the greatest in which the Greeks were ever involved, even greater than the Trojan War and the Persian Wars. He then explains the principles upon which he evaluates evidence; his basic perspective is that human nature is the basic cause of historical events (Thucydides attributes no historical event to either the gods or to fate). He declares that his History will not be so entertaining as some others (such as Homer and Herodotus), but instead be a rational analysis that will be useful to those who wish to understand the way things happen, since events similar to those of the past will certainly recur in the future because human nature is unchanging. He analyzes the events of this War, he tells us, in order to enable future generations to understand the causes and progress of future wars, though not necessarily to prevent them.
Next Thucydides explains the immediate causes of the War and gives an account of embassies and debates just before the War and the events of the first year (431), all of which are outlined in your textbook. Then he inserts the Funeral Oration given by the Athenian leader Pericles over the bodies of those who had died fighting for Athens. Partly to make his audience feel that their sacrifices are worthwhile, Pericles explains the nature of Athens's greatness, her freedom and democracy.
Thucydides sees how impressive human nature and life can be at their best, but also how rapidly both can degenerate under stress. The Melian Controversy gives the Athenian reasons for attacking the small island of Melos in 416, making them say bluntly that those who are powerful need have no regard for justice, human rights, or the gods. Compare this with the principles expressed in the Funeral Oration. Thucydides follows this with an account of the disastrous Athenian attempt to conquer the huge and resilient island of Sicily, a loss which, according to Thucydides, spelled the end of Athenian power.