Arms and the man
These were backed by services responsible for things like transport, supply, communications and medical support, although these did not really become part of the army proper until the 19th century was well under way.
'It was easier to move troops and to deliver physical shock in columns ...'
Success in battle often focused on the combination of the combat arms, with artillery preparing the way for attack by the infantry, and cavalry seeking intelligence, preventing a force from being surprised, charging on the battlefield and pursuing a beaten enemy.
Weapons were divided into those like longbow, musket, rifle and field gun, which achieved their effect by fire, and others, like pike, sword and bayonet, which accomplished physical shock. Often the two effects were merged. A line of infantry might charge after firing, with the implied shock inherent in its bayonets producing a psychological effect which caused its opponents to seek an urgent appointment elsewhere.
It was easier to move troops and to deliver physical shock in columns, formations which were deeper than they were broad, but to deliver fire in line. Armies moving into battle would deploy from column into line, a process in which drill and discipline were important.
Until the end of the 19th century, combat was a centralised process, with men fighting standing up, under the eye of their leaders and often within touching distance of their comrades. One of the consequences of the rise of the infantry rifle at the end of the 19th century was a more formless battlefield, with combatants more spread and often lying down. This tended to make formal discipline less useful than it had been in the past, and to emphasise individual qualities like initiative and determination.