Understanding IR 3 - WWII to the Cold War: Classical Realism

WWII, and all the more so the CW, were the historical context of the rise of classical realism within IR which itself moved from GB to the US and thus became an American social science, to quote S. Hoffmann.
Classical realists, and first and foremost Hans Morgenthau in his 'Politics among Nations', posit that international politics, like all politics, are power politics; they are state-centric in the sense they consider the unitary state, embodied in the statesman, as the only actor; statesmen are described as rational decision-makers eager to satisfy the national interest defined in terms of power; and since war as much as diplomacy is an instrument of foreign policy, at best a temporary stability, or international order, but not a genuine peace, is likely to be achieved thanks to a balanced distribution of material resources among major powers.
Updating past historians and theorists such as Thucydides, Machiavelli, Hobbes, and von Clausewitz, classical realism dominated IR theory during the two first decades of the CW. First because of an endogeneous reason - it correctly described the power political competition opposing the US to the USSR or, rather, it emphasized those features of this rivalry which were considered to be the most significant and which belied idealism. And secondly for an exogeneous reason: reralism provided what American statesmen needed, that is, a 'scientific' justification of America's planetary containment policy that put an end to its self-proclaimed exceptionalism.