Warfare and glory
This book begins with an explanation from Herodotus about why he's writing it, and the most important reason he lists is that he wants the world to remember the glory of battle between the Greeks and the Persians, and he wants for the people of Ionia to be remembered. This book goes through several wars among many people groups, and various battles are won and lost, either with honor, or without honor. The epic quality of the tales comes from the fact that these people treasured valiant warfare and bravery.
Human error and capriciousness
Although these stories are nearly unfathomable from a modern point of view, there are stories in the Histories that are thoroughly human, because they capture the flippant attitudes that people sometimes have. People in positions of great power of behave with pettiness, taking things personally, blowing things out of proportion. For the modern reader, this is a commentary on humanity itself, because these are still the problems that threaten our safety geopolitically, even though history has come so far since 400 BC.
The gods and fate
Fate is something that isn't really questioned in the book, but it is depicted thematically over and over again. When a person is made to lose in battle, there are often fateful reasons why that should be the case. Sometimes, the gods are explicitly mentioned, and the story even includes stories of supernatural encounters between men and gods in the course of human fate. In other words, the fate of men is attached to the will of the gods in Herodotus's point of view.
