Such were the influences which impelled Richelieu to make the rectification of the frontier of France on the side of the Netherlands, the Rhine, and the Pyrenees, the first object of his foreign policy; and to launch France on that career of conquest and aggrandisement at the expense of the House of Habsburg, which has been from his time almost to the present day the central feature of European politics. From the battle of Nördlingen to the battle of Solferino, there has hardly been a great war in Europe in which the armies of France and of the House of Austria have not been arrayed against each other as enemies. Spain was the first foe to be dealt with, for Spain was the most dangerous to neglect, and the easiest to attack. The Spaniards who garrisoned the Milanese had, in 1622, seized upon the valley of the Valtelline, and occupied it by force, in order to secure their communications with the Empire; and had even obliged Chur, the chief town of the League of the Grisons, to receive an imperial garrison. This was undoubtedly an act of aggression on their part, and gave Richelieu the opportunity of striking a deadly blow at his enemy. The Valtelline is a broad and rich valley which runs in a north-easterly direction into the heart of the Rhaetian Alps from the top of the Lake of Como. About half-way up the valley a mountain pass, practicable for the passage of troops, leads to the east into the valley of the Adige a little north of Trent, from which by the well-frequented Brenner Pass communication with Innsbrück and south Germany was easy and safe. This was the only route which was certain to be available for the passage of troops and stores from the Empire to Milan, as the other mountain passes, which led direct from Tirol and Carinthia into Italy, opened into the territory of the republic of Venice, and Venice was usually not inclined to welcome the arrival of imperial troops. Provided, however, that the passage of the Valtelline was secured, the rest of the way was safe, as it lay through imperial territory. Hence the command of the Valtelline was absolutely essential to the maintenance of the power of the Habsburgs in Italy, but the valley itself was politically subject to the League of the Grisons, which as long ago as 1509 had come under the protection of France. So then, when Spain moved troops into the Valtelline, built a fortress in the valley, and obliged the Grisons to admit an imperial garrison at Chur, Louis XIII. as the protector of the Grisons had the right to interfere. [Sidenote: =Its recovery for the Grisons, 1626.=]