What did not develop so soon was the fact that this new invasion was to be under the leadership of the German General von Mackensen, and that the invaders were to consist in large part of German regiments. During the summer Mackensen had been engaged in directing a strong Austro-German offensive against the Russians, with conspicuous success. For weeks after he had left this front and was busy organizing a similar offensive against the Serbians, the German official dispatches continued to associate his name with actions on the Russian front that the preparations in the south might continue secret as long as possible. Not long after the first Austro-German guns began hurling their shells across the Danube, against the Serbian position at Semendria, the Serbians learned of the disposition and the resources of the enemy. The troops under Mackensen were divided into two armies, each in close contact with the other. One of these wings was under the command of a German, General von Gallwitz, who had distinguished himself against the Russians a short time previously. The men under him were entirely Germans. The other army was under the command of an Austrian, General von Koevess von Koevesshaza. His men were both German and Austrian, the latter predominating. The army under Gallwitz extended from Orsova, near the Rumanian frontier, along the Danube westward to a point opposite Semendria. Here his right flank joined Koevess's line, which extended up past Belgrade, along the Save and part way up the Drina. The rest of the frontier up the Drina was covered by a smaller Austrian army. Altogether, the Austro-German armies comprised at least 300,000 men. The Austrians were picked troops, for it was only natural that the general staff wished to retrieve, in some measure, the humiliation of the previous year. The Germans, numbering fully half of the total force, were also hardened veterans, who had seen plenty of fighting on the Russian front or in France or Flanders. Mackensen's overwhelming success in driving the Russians out of Galicia had been mainly due to his artillery, that arm of the military service in which the Germans excelled all their enemies. And here, too, the artillery was to play an important part, for fully 2,000 cannon, nearly all of mid-caliber and heavy caliber, had been brought down against the Serbians. During the first three invasions the Austrians had thrown their infantry up against the Serbian lines. Now German tactics were to be tried: the Serbian trenches and other defensive positions were to be pulverized with powerful explosives, then rushed with infantry.