Xenophon does not get enough credit as a historian in view of the greatness of the history of Thucydides and Herodotus. However, Xenophon is a key history of his day and age. Also his work is the primary one of the end of the Peloponnesian War. His history starts in 411, and he goes all the way to 362, the battle of Mantineia.
In terms of a summary, Xenophon describes many central events. Here is a list of many of them: the destruction of the Long Walls of Athens, the surrender of Samos, the Thirty Tyrants, Sparta's Asiatic campaigns, the Corinthians War, the King peace, Spartan imperialism, the Battle of Leuctra, and many more important events and battle in the Greek history.
As one can see Xenophon tries to continue the narrative of Thucydides. It should also be said that he is not a historian that is widely read, but for anyone wanting information about this time period, he is essential.