Begin with a clear reason to keep reading: a vivid moment, a useful question or a concise statement of the challenge the book addresses. Then orient the reader. They should understand whose perspective they are following, what kind of book this is and what value the next chapters will provide.
Resist explaining the entire history at once. Introduce only the context necessary for the opening to make sense, and let later chapters supply deeper background. During editing, check that the first pages deliver on the promise in the title and description rather than delaying the subject readers came for.