These "Saracens", as they were known, blocked the Alpine passes to Christian travelers until their expulsion by Christian forces led by Arduin Glaber in 973, at which point transalpine trade was able to resume. Not until the final breakup of the Carolingian Empire in the 10th and 11th centuries is it possible to trace out the local history of different parts of the Alps, notably with the High Medieval Walser migrations. The French historian Fernand Braudel, in his famous volume on Mediterranean civilisation, describes the Alps as “an exceptional range of mountains from the point of view of resources, collective disciplines, the quality of its human population and the number of good roads.” This remarkable human presence in the Alpine region came into being with the population growth and agrarian expansion of the High Middle Ages. At first a mixed form of agriculture and animal husbandry dominated the economy. Then, from the Late Middle Ages onwards, cattle tended to replace sheep as the dominant animals. In a few regions of the northern slope of the Alps, cattle farming became increasingly oriented toward long-range markets and substituted agriculture completely. At the same time other types of interregional and transalpine exchange were growing in significance. The most important pass was the Brenner, which could accommodate cart traffic beginning in the fifteenth century. In the Western and Central Alps, the passes were practicable only by pack animals up to the period around 1800. The process of state formation in the Alps was driven by the proximity to focal areas of European conflicts such as in the Italian wars of 1494–1559. In that period the socio-political structures of Alpine regions drifted apart. One can identify three different developmental models: one of princely centralization (Western Alps), a local-communal one (Switzerland) and an intermediate one, characterised by a powerful nobility (Eastern Alps). Until the late nineteenth century many Alpine valleys remained mainly shaped by agrarian and pastoral activities. Population growth favoured the intensification of land use and the spread of corn, potato and cheese production. The shorter growing season at higher altitudes did not seem to be an impediment until around 1700. Later, however, it became a major obstacle to the further intensification of agriculture, especially in comparison to the surrounding lowlands where land productivity increased rapidly. Inside the Alpine region there was a striking difference between the western and central parts, which were dominated by small farming establishments, and the eastern part, which were characterised by medium or big farms.