In these encounters, as during later battles of the campaign, the French troops discovered a cohesion and steadiness and a military habit of discipline assumed to be foreign to their temperament. But their units had been trained to act together in masses on practical lines. Of the value of that training General Joffre was well aware. He knew also that success in the earlier encounters, which that training would go far to ensure, must give his troops an invaluable confidence in their own quality. There were, however, two surprises even more marked. One of these was the quite unexpected use made of the fortified frontier; the other, associated with it, was that of allowing the Germans to advance upon Paris with an insufficient force, in the belief that French movements were being conformed to their own. Undoubtedly as regards the fortified frontier the belief prevailed that the chief difficulty would be that of destroying its works with heavy guns. It had never been anticipated that the Germans might be prevented from getting near enough for the purpose. But in the French strategy Verdun, Toul, and Belfort were not employed as obstacles. They were employed as the fortified bases of armies. Being fortified, these bases were safe even if close to the scene of operations. Consequently the lines of communication could be correspondingly shortened, and the power and activity of the armies dependent on them correspondingly increased. So long as these armies remained afoot, the fortresses were unattackable. Used in that way, a fortress reaches its highest military value. The strategy adopted by General Joffre in association with the German advance upon Paris is one of the most interesting phases of the war. His _tactics_ were to delay and weaken the first and driving formation of the German chain of armies; his _strategy_ was, while holding the tail of that chain of armies fast upon the fortified frontier, to attract the head of it south-west. In that way he at once weakened the chain and lengthened out the German communications. Not merely was the position of the first German army the worse, and its effective strength the less, the further it advanced, thus ensuring its eventual defeat, but in the event of defeat retirement became proportionally more difficult. The means employed were the illusion that this army was driving before it, not a wing of the Allied forces engaged merely in operations of delay, but forces which, through defeat, were unable to withstand its march onward. It cannot now be doubted that the Germans had believed themselves strong enough to undertake the investment of Paris concurrently with successful hostilities against the French forces in the field. But by the time General von Kluck's army arrived at Creil, the fact had become manifest that those two objectives could not be attempted concurrently. The necessity had therefore arisen of attempting them _successively_.