Students of political systems grapple with a subject matter that is today in constant flux. They must deal not only with the major processes of growth, decay, and breakdown but also with a ceaseless ferment of adaptation and adjustment. The magnitude and variety of the changes that occurred in the world’s political systems beginning in the early 20th century suggest the dimensions of the problem. Great empires disintegrated; nation-states emerged, flourished briefly, and then vanished; world wars twice transformed the international system; new ideologies swept the world and shook established groups from power; all but a few countries experienced at least one revolution and many countries two or more; domestic politics in every system were contorted by social strife and economic crisis; and everywhere the nature of political life was changed by novel forms of political activity, new means of mass communication, the enlargement of popular participation in politics, the rise of new political issues, the extension of the scope of governmental activity, the threat of nuclear war, and innumerable other social, economic, and technical developments.
