Howard Zinn's The Politics of History goes through some major historical events as they relate to issues of justice for ordinary Americans and the U.S. government's foreign policy. The other part of the book is on how to write history. It is a standard trope that the sciences are to be objective, but Zinn believes this ought to be clarified.
According to Zinn, the sciences tend to have an interest in why something occurs and being able to predict future instances. We ought to be as objective as possible about the evidence but it is fine to pursue some knowledge for some purpose. As Alfred North Whitehead wrote, "Knowledge keeps about as well as fish."
In the context of history and writing about history, it is fine to focus on a particular issue for humanitarian reasons. In fact, that kind of knowledge is all the more relevant to how we live. For instance, think of the Vietnam War. The war was essentially a revolutionary war among the Vietnamese and the U.S. government interfered with it. We can also examine the motivations and flimsy evidence that the government used to justify the invasion. This will help us explain the Iraqi War and other justifications for war.
This is a good book, one that all historians and aspiring historians ought to read. Or anyone, really.