The formation of the European state system
Most of the central part of the European Continent consists of flat land, increasingly cultivated from the last centuries of the first millennium onwards, with no obvious divisions in the terrain. The transition from the first to the second phase in the military development was least partly a response to the various invasions of Europe from the seventh century onwards, the Arabs in the south and the Scandinavians and Slavs in the north and east. Despite the superiority of defence over attack, however, some major changes between the main powers of Europe took place in the thirteenth century. In contrast to other parts of the world, such as the Arab and Ottoman states and empires, there are relatively few examples in Europe of kingdoms formed by conquest. The Peace Treaty of Westphalia of 1648 is often regarded as the precise date when the European state system came into existence, including the around 400 German territories.