Probably believing that the British army had been hurried to the aid of General Sarrail, General von Kluck advanced confidently. Having concealment in view, the commanders of the French army and the British army between them had left a wide gap between the two armies. Through one of these apparently unguarded openings a strong body of uhlan patrols advanced, riding southward until they reached Nogent, south of Paris, and seemingly with the whole rich country of central France laid wide open to a sharp and sudden attack. Among the many strange features of this series of the battles of the Marne this must certainly be reckoned as one. Though possessing an unequaled military organization, though priding itself on its cavalry scouts, though aided by aerial scouts, and though well supplied with spies, yet the Allied armies, with the age-old device of a forest, were able to cloak their movements from this perfectly organized and powerful invading army. Much of the credit of this may be assigned to the French and English aircraft, which kept German scouting aircraft at a distance. But the Allied generals were astounded at the result of their maneuver, which, as they admitted afterward, was merely a military precautionary measure against the discovery of artillery sites, and a device to keep the enemy in general ignorance. On Saturday, September 5, 1914, at the extreme north of the line of the two armies facing each other across the Ourcq, an artillery duel began. The offensive was taken by the French, and though in itself it was not more striking than any of the artillery clashes that had marked the previous month's fighting, it was significant, for it marked the beginning of the battles of the Marne. The plans of General Joffre were complete, but the actual point at which the furious contest should begin was not yet determined. In the northern Ourcq section, however, the realization by the French that they were actually on the offensive at last, that the long period of retreat was over, could not be restrained. The troops were eager to get to work with the bayonet, and greatly aided by their field artillery, in which mobility had been sacrificed to power, they quickly cleared the hills to the westward of the Ourcq. By nightfall of September 5, 1914, the country west of the Ourcq was in French hands. But to cross that river seemed impossible. General von Kluck's heavy artillery had been left behind to hold that position, and every possible crossing was covered with its own blast of death.