The tactics of a phalanx was introduced into Rome from Etruria in the sixth century. After Rome incorporated Etruria, she contended against Gallia and Samnium, and penetrated into the weak points of a phalanx in the fourth century. The weaknesses were that the defeat of a part of files was connected the collapse of a whole formation, that a phalanx was restricted by the lay of the land, and that a mobility lacked because the legionaries were standing densely in the phalanx. The organization for overcoming the weaknesses was a maniple divided velites, hastati, principes, and triarii. The battle method of a maniple was a sequence attack from the velites took charge of a combat in outpost to the triarii composed of the veterans. Therefore, the defeat of a part of files was not connected the collapse of a whole formation. And a maniple could deploy in a small area.
Marius reorganized a maniple into a cohort. The peasant fell by the development of latifundium and the supply of low-priced slaves. The decrease of the peasant-soldier meant the weakening of military strength. Marius permitted the plebs without property enlisted, in order to increase the number of the legionaries. The advantages of a cohort were a commander could try diverse tactics because all of the legionaries equipped the same armament, could convey quickly an order to the legionaries, and could utilize the legionaries with having wide experience. And the relationship between the legionaries was strengthen in a cohort.
The tactical formation of the Roman Army was changed by the condition of enemies, or by economic situation. Rome didn't stick to one organization, but carried out diverse systems for improving the weaknesses of existing organization. An important factor of success of Roman Army was an attitude of the Romans that was trying to improve the weaknesses, and to realize diverse methods by the existing state of things.