Chapters of history can refer to wars and ERAS. Wars are periods of armed conflict between two or more countries, while ERAS are longer periods of time that are usually divided into smaller parts. Wars often define the boundaries of an era, such as the World Wars of the 20th century. ERAS can also be defined by cultural, political, or economic changes, such as the Industrial Revolution. In this context, wars and ERAS are both important chapters of history.