In case of an attack they retire slowly, and on joining the main body, take their assigned position in the line of battle.
In an open country the order of march presents but little difficulty; but in a broken country, and especially in the vicinity of the enemy, a march cannot be conducted with too many precautions.

Before engaging in a _defile_ it should be thoroughly examined, and sufficient detachments sent out to cover the main body from attack while effecting the passage.

A neglect of these precautions has sometimes led to the most terrible disasters.
In military operations very much depends upon the rapidity of marches.
The Roman infantry, in Scipio's campaigns in Africa, frequently marched a distance of twenty miles in five hours, each soldier carrying from fifty to eighty pounds of baggage.

Septimius Severus, Gibbon states, marched from Vienna to Rome, a distance of eight hundred miles, in forty days.

Caesar marched from Rome to the Sierra-Morena, in Spain, a distance of four hundred and fifty leagues, in twenty-three days! Napoleon excelled all modern generals in the celerity of his movements.
Others have made for a single day as extraordinary marches as the French, but for general activity during a campaign they have no rivals in modern history.