This introduction presents an overview of key concepts covered in the subsequent chapters of this book. The book looks at the kinds of things that armies door are prepared to do. Unlike, say, police forces, most European armies, with the exception of those relatively small elements that are deployed overseas, spend a great deal of their time preparing to do their job. Germany had large numbers of armored forces and great numbers of reserve forces; both have nearly disappeared in the post-Cold War period. In addition to homeland defense, European military forces have always been heavily involved in the second homeland mission, that of civil support. The book also looks at the employment of military forces in a domestic environment. It contains domestic roles of military forces; a role might be to provide support to the civil authority, which we refer to as Defense Support to Civil Authority (DSCA).