From the Middle Ages onward, French rulers believed their kingdoms had natural borders: the Pyrenees, the Alps and the Rhine. This was used as a pretext for an aggressive policy and repeated invasions. The belief, however, had little basis in reality for not all of these territories were part of the Kingdom and the authority of the King within his kingdom would be quite fluctuant. The lands that composed the Kingdom of France showed great geographical diversity; the northern and central parts enjoyed a temperate climate while the southern part was closer to the Mediterranean climate. While there were great differences between the northern and southern parts of the kingdom there were equally important differences depending on the distance of mountains: mainly the Alps, the Pyrenees and the Massif Central. France had important rivers that were used as waterways: the Loire, the Rhone, the Seine as well as the Garonne. These rivers were settled earlier than the rest and important cities were founded on their banks but they were separated by large forests, marsh, and other rough terrains.