On the military side the Serbian campaign was the simplest of operations. For many months the Serbian forces had been posted south of the Danube and the Save and east of the Drin, looking over their frontiers into Hungary and Bosnia. Behind them from the Danube at Belgrade to the Aegean at Saloniki ran the Orient railroad, by which they were munitioned. At Nish halfway to the sea, the line drew near to the Bulgarian frontier and sent a branch off, which passed through Bulgaria and reached Constantinople. The Saloniki railway was the life line of Serbia, it was also the natural route for a retreat, if the Austro-German attack became too heavy. But it was fatally exposed, should Bulgaria enter the war against Serbia. In the Treaty of Bucharest, Greece and Rumania had undertaken to join Serbia should she be attacked by Bulgaria, and the mission of Greece was to cover the Saloniki railroad as far north as it was necessary to join hands with the Serbians. Now, while the Bulgarians were beginning to mobilize and the Austro-German hosts were gathering to the north, Serbia appealed to her former allies to keep their agreement. Both declined, and their refusal was fatal. The Allies had relied upon Greek promises, and had failed to collect any considerable force at Saloniki. They had trusted Bulgaria and refused to let Serbia attack her neighbor before Bulgarian mobilization was complete. Once Bulgaria had mobilized the doom of Serbia was settled. What happened was this: The Germans forced the passage of the Danube north and east of Belgrade and came south along the broad Morava River Valley, driving the Serbs before them. Thanks to the heavy artillery of the invaders Serbian resistance was impotent. The Austrians, meantime, crossed the Drin and came east from Bosnia. Think of Serbia as a rectangle and you can visualize two sides of the figure as closing in on the center, which was the heart of Serbia. At the appointed moment the Bulgarians struck west from a third side of the rectangle, speedily crossed the Belgrade-Nish-Saloniki railroad, and thus cut off the true line of Serbian retreat, that upon Saloniki. Very early in the campaign the Bulgars seized Uskub, thus interposing a wedge between the small Anglo-French force at Saloniki and the Serbs about Nish to the north of Uskub. Meantime a desperate concentration was taking place at Saloniki, and an Anglo-French force, commanded by Sarrail, was being pushed up the Saloniki railroad toward Uskub to open a road to the Serbs to join their allies. The operation suggested that successfully conducted in Flanders in the opening months of the war, which enabled the Belgian army to escape from Antwerp and join their allies in Flanders.