If from this position it advance its left, possessing itself of the kingdom of Valencia, the line of the Sierra d'Estellas becomes its third base of operations against the centre of Spain.
A base may be parallel, oblique, or perpendicular to our line of operations, or to the enemy's line of defence.

Some prefer one plan and some another; the best authorities, however, think the oblique or perpendicular more advantageous than the parallel; but we are not often at liberty to choose between these, for other considerations usually determine the selection.
In 1806, the French forces first moved perpendicular to their base on the Main, but afterwards effected a change of front, and moved on a line oblique or nearly parallel to this base.

They had pursued the same plan of operations in the Seven Years' War.

The Russians, in 1812, based perpendicularly on the Oka and the Kalouga, and extended their flank march on Wiozma and Krasnoi; in 1813, the allies, based perpendicularly on Bohemia, succeeded in paralyzing Napoleon's army on the Elbe.
An American army moving by Lake Champlain, would be based perpendicular on the great line of communication between Boston and Buffalo; if moving from the New England states on Quebec and Montreal, the line of operations would be oblique; and if moving from the Niagara frontier by Lake Ontario and the St.Lawrence, the line would be nearly parallel both to our base and to the enemy's line of defence--an operation, under the circumstances, exceedingly objectionable.
Any point in the theatre of operations which gives to the possessor an advantage over his opponent, is regarded as _strategic_.

Their geographical position and political and military character, give them a greater or less influence in directing the campaign.