World War I was actually the culmination of the rivalries of the nation-states of Europe. All throughout the 1800's Britain had been expanding its Empire; many of the Continental countries, especially Germany, were busy integrating domestically and colonizing abroad. When confilict finally came between the European players, nationalism was at its peak -- the states of Western Europe fought each other primarily because each perceived itself to be better than its neighbor. The thinking then, for example, was that a Englishman would become a German once the Germans pointed out the deficiencies of English ways. So Nationalism all around was extreme. When the fighting came, it only intensified. Within the British Empire, the perception of Mother England being in trouble with its too few soldiers within Britain brought much of the Empire's manpower to bear in the War.