Line of Operation
(The) in war. The line between the base of operation (q.v.) and the object aimed at. Thus, if a fleet is the base and the siege of a city is the object aimed at, the line of operation is that drawn from the fleet to the city. If a well-fortified spot is the base and a battle the object, the line of operation is that which lies between the fortified spot and the battle-field.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894