Hans Morgenthau is, together with E.H.Carr, among the key 20th century thinkers who studied international relations and developed what is today called classical realism (as opposed to structural- or neo-realism - see Kenneth Waltz). Classical realism claims an ancient ancestry beginning with the History of the Peloponesian War by Thucydides, and including thinkers like Maciaveli and Clausewitz, as well as Ralf Niebur. Briefly, the proponents of this line of thinking claim that 1) states are the only actors of significance in international relations; 2) that because there is no single world government, international relations are characterized by self-help- if states wish to survive they should ever be ready for war; 3) 'power' in this sense becomes both a means (to survival) and an end in itself -- states struggle to aggrandize their power; 4) because of all this, international relations are primarily a realm of conflict, not cooperation; 5) this sorry state of affairs stems from the evil nature of man which is essentially unchangeable (see St. Augustine, City of God, for a forceful argument in this vein). Given all this, Morgenthau claims that any politician who does not take the above points seriously is acting under dangerous delusions. Because most statesmen do act in this way, he, says, reading his book will allow the reader to be able 'to look over the shoulder' of any leader and understand their decisions. Whether one agrees or not with these views, the book is indispensible for any serious student of international politics. For further interest one may wish to read both scholars who have advanced the realist tradition in international relations in new directions (see esp. Kenneth Waltz, Theory of International Politics; Robert Jervis, Perception and Misperception..., and System Effects; Jack Snyder, Myths of Empire; and Michael Doyle, Ways of War and Peace) and critics of this tradition (esp. Robert Keohane, Neorealism and Its Critics, After Hegemony, and (with J.Nye) Power and Interdependence, as well as David Baldwin, ed., Neorealism and Neoliberalism). I admit that the book is densely written, but with the right roadmap, it is an enjoyable read. In fact, if you wish to find it even more useful, you may begin with E.H.Carr, The Twenty Years' Crisis, which is elegantly written, a real pleasure to read, and which is a very clear statement of the main arguments of realism.