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 Kövess von Kövesshaza. 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 Kövess'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.
Though they had been undisturbed for so long, the Serbians were by no means in doubt as to what was yet to come. They had realized that eventually the enemy would return more determined and more powerful than ever. Therefore, they had spent the nine months since the last defeat of the Austrians in extensive preparations. Line after line of trenches had been built back into the interior of the country, and all the possible crossings on the rivers had been heavily fortified. Moreover, they had drained the civilian population of every male person strong enough to carry a gun.
At this time, when the fourth invasion began threatening, their army mustered fully 310,000 men, slightly more than the Austro-German. In regard to small arms and ammunition they were also at least equal to the enemy, for vast consignments of military stores had been sent into the country by the Allies. Only in heavy artillery were they inferior, but then this was also true of all the armies facing the Germans throughout Europe.
Therefore, had the Serbians been called upon to defend themselves only against General von Mackensen's armies, it is highly probable that they would have been able to give the same answer as they had the year previous. So probable, in fact, that Mackensen would hardly dared to have attacked them with only 300,000 men. To be sure, their enemy was no longer made up of raw recruits and there was now the heavy artillery as well as a commander of great ability to face, but the preparations they had made in defensive works, as well as the mountainous nature of their country, more than made up for these advantages possessed by their opponents. It was the Bulgarians who would turn the scale.
Because of the greed for territory of their governing clique, the Serbians now faced dangers which even their rugged qualities could not contend against long. For now, while they were steeling themselves to meet the impact of the blow from the Austro-Germans from the north, the Bulgarian army, fully as strong as themselves, was gathering on their right flank. In spite of the diplomatic protests of Ferdinand and Radoslavov, the Serbians were not deceived.
The danger from the Bulgarian army meant more to the Serbians than the mere doubling in number of their enemy's forces. It was the position of the Bulgarians which made the situation especially precarious, impossible.