Because the principles of international relations do not change, even if the settings do. Writing about Kissinger, Robert D. Kaplan noted that
if the technology of war had changed, Kissinger implied, the task of statesmen remained the same: to construct a balance of fear among great powers as part of the maintenance of an orderly international system - a system that, while not necessarily just or fair, was accepted by the principal players as legitimate. As long as the system was maintained, no one would challenge it through revolution - the way Hitler in the 1930's.
Metternich is worthy of study: if peace is the goal of a diplomat, Metternich excelled. The Congress of Vienna created a stable system that ensured peace for a century.
With the British Foreign Secretary, Viscount Robert Stewart Castlereagh, Metternich built an order so ingenious that from 1815, the year of Napoleon's defeat at Waterloo, to the outbreak of the First World War, a hundred years later, Europe knew no major conflicts, with the exception of the ten-month-long Franco-Prussian War, in 1870-1871. Thanks in significant measure to Metternich, who did everything in his power to forestall the advent of democracy and freedom in the Hapsburg and Ottoman Empires, Europe in 1914 saw peace and steady economic growth as natural and permanent conditions. Europe had thus lost that vital, tragic sensibility without which disaster is hard to avoid, and troops rushed onto the battlefields of Flanders in a fit of romanticism.
In addition to the Franco-Prussian War, there was also the Crimean War, and a few other conflicts - all of which, added together, were laughably small, compared two the horrors which flank the century of peace: the 25 years of French Revolution and Napoleonic Wars at the beginning, and World War One at the end.
Although Kissinger's study of Metternich, published as a book with the title A World Restored, is in no way about Hitler, the generalizations which he makes seem to implicitly reflect Kissinger's experience of Germany in the 1930's:
Kissinger's response to Munich and Nazism in A World Restored is pellucid. The key word is "revolution," something that Kissinger's experience as a youth, augmented by scholarship, taught him to fear. Rapid social and political transformation leads to violence, whether throughout the Europe of the early 1800's, owing to Napoleon's aggression -- itself a direct result of the French Revolution - or in the Germany of the 1930's. Although the word "revolution" is applied to the America of the 1770's ... the cultural and philosophical awakenings among English settlers in America ... took place over decades and were, in truth, evolutions. Iran did experience a revolution in the late 1970s, as did Cambodia in 1975, China in the late 1940's, and Russia in 1917.Kissinger saw, as did Edmund Burke and others, that the American Revolution and the French Revolution were fundamentally and essentially different - and latter marked out a path to later be taken by Hitler, Stalin, Mao, and Pol Pot.