It has been frequently noted throughout history that the imposition of a foreign threat or foreign rule on a people has led to the solidification of the national identity and spirit. Frequently we have seen those that are closest to one another in terms of culture, background, language or religious tenets dispute with one another vociferously on what can, from the standpoint of history, be seen as relatively minor differences. In many cases this has prevented a national unity from arising spontaneously, and it has only been the imposition of a foreign yoke or domination that has caused these people to recognize their oneness and join forces to defeat, or in some cases assimilate, the common enemy.
History is replete with examples, as Sri Aurobindo notes: “There is none of the modern nations in Europe which has not had to pass through a phase more or less prolonged, more or less complete, of foreign domination in order to realise its nationality. In Russia and England it was the domination of a foreign conquering race which rapidly became a ruling caste and was in the end assimilated and absorbed, in Spain the succession of the Roman, Goth and Moor, in Italy the overlordship of the Austrian, in the Balkans the long suzerainty of the Turk, in Germany the transient yoke of Napoleon. But in all cases the essential has been a shock or a pressure which would either waken a loose psychological unity to the necessity of organising itself from within or would crush out, dispirit or deprive of power, vitality and reality the more obstinate factors of disunion. In some cases even an entire change of name, culture and civilisation has been necessary, as well as a more or less profound modification of the race. Notably this has happened in the formation of French nationality. The ancient Gallic people, in spite of or perhaps because of its Druidic civilisation and early greatness, was more incapable of organising a firm political unity than even the ancient Greeks or the old Indian kingdoms and republics. It needed the Roman rule and Latin culture, the superimposition of a Teutonic ruling caste and finally the shock of the temporary and partial English conquest to found the unequalled unity of modern France. Yet though name, civilisation and all else seem to have changed, the French nation of today is still and has always remained the old Gallic nation with its Basque, Gaelic, Armorican and other ancient elements modified by the Frank and Latin admixture.”
Sri Aurobindo, The Ideal of Human Unity, Part I, Chapter 5, Nation and Empire: Real and Political Unities, pp. 38-39