All modern armies are organized on the same basis.  They are made up of a Staff and Administrative departments, and four distinct arms—­Infantry, Cavalry, Artillery, and Engineers; each having distinct duties, but all combining to form one and the same military body.  In the actual operations of a campaign, these forces are formed into corps d’armee, each corps d’armee being composed of two or more grand-divisions; each grand-division, of two or more brigades; and each brigade, of several companies, squadrons, or batteries.

In speaking of an army in the field, it is sometimes supposed to be divided into two classes of men—­the Staff and the line.  We here include in the first class—­

All officers, of whatever arm, above the rank of colonel;

All officers of the staff corps of whatever grade, and

All officers attached to the staff as aides, &c.;

All officers of the administrative departments;

All officers of artillery and engineer staffs;

The corps of geographical or topographical engineers, and

The guards.

In the second class are included all troops, of whatever arm, which belong to the active army, in infantry, cavalry, artillery, and engineers.  All troops on detached service, such as recruiting, guarding posts and depots, escorting convoys, &c., as well as all sedentary corps, garrisons of fortified places, &c., are not regarded in this classification as composing any part of the line of the army.

Troops of the line is a term applied only to such troops as form the principal line on the battle-field, viz:—­The heavy infantry and heavy cavalry.  These are technically called infantry of the line, and cavalry of the line.  In this sense of the term, light infantry, light cavalry or dragoons, artillery, and engineers, are not classed as troops of the line.  But this distinction is now pretty much fallen into disuse, and the division of an army into Staff and Administrative departments, and four arms of service—­Infantry, Cavalry, Artillery, and Engineers—­is now regarded as the most convenient, from being precise and definite in its meaning.

The general staff of an army includes all general officers of the army, and such officers of lower grades as are attached to this general duty, instead of serving with troops, or on special administrative duty.  The general officers are—­1st, the generalissimo, or commander-in -chief; 2d, generals, or marshals, as they are called in France, or field-marshals and generals of infantry and cavalry, as they are called in England and the northern states of Europe; 3d, lieutenant-generals; 4th, generals of division, or major-generals, as they are called in England; 5th, generals of brigade, or brigadier-generals, as they are sometimes called;—­colonels,