Strategy has been defined by military writers as the 'science of generals,' 'the art of making war on the map,' or 'the art of rightly directing masses of troops towards the object of the campaign;' it is dependent upon the due consideration of everything that can possibly influence the campaign. Roughly speaking, strategy directs the movements of troops until contact with the enemy is imminent. From that moment all combinations and manoeuvres are classed as Tactics (q.v.), until perhaps the opposing armies become again sufficiently separated for strategy to be employed. The object of strategy is to bring an adversary into such a position that the chances of victory will be against him, and defeat will entail disasters beyond the loss of the battle. Strategy will be offensive or defensive according to political or geographical considerations and the relative strength or mobility of the belligerents. The former will give all the advantages of the initiative to the commander who can adopt it. He will by invading the enemy's country consume his supplies, and spare his own the horrors of war. He will be able to make and carry out his plans unimpeded by his opponent, who will be ignorant where the mass of his troops are concentrated, and so must await the attack in a more or less scattered and therefore dangerous condition. Defensive strategy, on the other hand, has advantages in facility of supply and transport, freedom of movement, and power to utilise obstacles. Also a defender becomes stronger as he retires, whilst his assailant grows weaker as he advances, and must leave troops behind him or bring fresh forces into the theatre of war in order to guard his base of operations, where his supplies collect, and the lines of communication by which they reach him.
The assailant will endeavour to reach his objective point—i.e. some place, generally the capital city, the capture of which will end the campaign or enable him to make a further advance. His troops must be so disposed as to be able to concentrate on important points in numbers superior to the enemy. The latter, on the other hand, will endeavour to do the same, and also to operate against his adversary's communications without exposing his own. As he probably can shift his base and lines of communication more easily than the invader, some advantage will here accrue to him. Also, his troops being generally more concentrated, he can probably act on interior lines—e.g. if of four equidistant armies three were on the circumference of a semicircle and the fourth at the centre, it is evident that the latter might defeat any one of the former before it could be reinforced by either of the others.
Perhaps the most brilliant example of this was shown by Napoleon I. in the first part of the campaign of 1814. Towards the end of January of that year he with 70,000 young conscripts was at Châlons; to the south 160,000 Austrians and Russians under Schwarzenberg were advancing from Basel along the valley of the Seine, and to the north 60,000 Prussians under Blücher along the Marne from Mannheim, the objective of both being Paris. Napoleon after the indecisive battle of Brienne was defeated by Schwarzenberg at La Rothière. The latter moved slowly, and Blücher, thinking to gain Paris first, moved his corps by several roads. Napoleon leaving 20,000 men under Oudinot and Victor to hold the passages over the rivers, and to delay Schwarzenberg still further, carried the remainder rapidly against the scattered Prussians, defeating them in detail at Champaubert, Montmirail, and Château-Thierry, obliging Blücher to retire to Châlons. He then turned upon Schwarzenberg, beat him, and had driven him back to Troyes by the end of February. The allies were so dispirited that they asked for an armistice.
See Bigelow's The Principles of Strategy (1891); the Letters on Strategy by Prince Kraft zu Hohenlohe-Ingelfingen (trans. 1898); and works cited at TACTICS.