Ottoman expansion toward Vienna followed the Danube as a supply route. An army of the size being deployed needed large quantities of supplies that could move up the river. Control of cities and chokepoints along that route determined whether those supplies could get through. An advance could go no farther than the reach of a supply system capable of sustaining it. The river therefore helps explain both the direction of Ottoman campaigns and the limits they encountered.
Chapter · 8:58-9:43
Danube chokepoints limited Ottoman advances
Ottoman campaigns toward Vienna depended on controlling the Danube points through which army supplies had to pass.