<P> The military convention between Bulgaria and the Central Powers laid down the general plan for its campaign against the Kingdom of Serbia . It severely limited the control of the Bulgarian High Command over the Bulgarian 1st Army , which was designated part of a combined German , Bulgarian and Austro - Hungarian force commanded by Field Marshal August von Mackensen . He had recently led the German and Austro - Hungarian armies in the highly successful and victorious Gorlice -- Tarnów Offensive of the Central Powers against the Russian army on the Eastern Front . His army group was created specifically to wage war against the Serbian army in the pre-1913 borders of the country ( `` Old Serbia '' ) , to defeat it wherever it found it and to open the land route between Hungary and Bulgaria . As commander , Mackensen acted independently and received his directives only from the German High Command . However , the field marshal 's orders to his Bulgarian forces had to be relayed to the commander of the 1st Army by the Bulgarian General Staff , which left room for the latter to intervene when needed . According to the convention , the Bulgarian commander - in - chief retained full and direct control over the Bulgarian 2nd Army and its operations in Vardar Macedonia . </P>
who did the central powers defeat on the eastern front before concentrating back on france
the Russian army