In order to insure immediate and full effect to these articles, the provisions respecting guarantees of the whole treaty involve the occupation of this very region. German territory west of the Rhine, together with the Rhine bridgeheads, is to be held for fifteen years by Allied and Associated troops. In case of faithful execution of the treaty, this region is to be evacuated by these troops in three successive zones at intervals of five years; in case of non-execution, the territory may be reoccupied and the period of occupation extended.[36] A further agreement, of even date with the treaty, provides for the administration of the occupied territory under a civilian Inter-Allied Rhineland High Commission representing France, Belgium, Great Britain, and the United States, subject to whose authority the German local administration is maintained. Neither the military occupation nor the civilian Commission covers the demilitarized zone on the Right Bank, a gap between the two systems of administration which was to prove particularly serious in the region of the Ruhr, the principal source of the coal on which France and other Allies had an option under other clauses of the treaty, a district liable to serious industrial disturbances for the suppression of which the German government would demand the right to use troops. Finally, as a more positive and direct guarantee of the country which had borne the brunt of Germany’s aggression, Great Britain and the United States agreed to come to the aid of France in case of an unprovoked attack by Germany.[37] Designed to offer adequate assurance during the transitional period while the League of Nations was getting under way, this supplementary treaty recognized not only the peculiar dangers of France, exposed directly to the full force of a German offensive, but also the general interest in her full security and protection. To the French this was an essential part of the peace settlement, and without it they would have insisted on more direct and more material guarantees of their own. * * * * * Another matter affecting the Left Bank came into prominence at the conference, namely the valley of the Saar. From one point of view this was a phase of the question of Alsace-Lorraine, for a portion of the Saar basin had once been a part of Lorraine and the recovery of the lost provinces revived the question of their historic boundaries. It was also part of the problem of the Left Bank, for the territory belonged to Prussia and Bavaria and was inhabited by a population of predominantly German affinities, and any annexation here was subject to the same objections as elsewhere on German soil. Lastly, the coal mines of the valley raised a more special question, for they adjoined immediately the new boundary of France, and thus offered an easy source of reparation for the destruction and devastation of French territory.