2 armies marched towards a small town in Virginia or Tennessee. Their Scouts see each other at a distance.
Now what?
How do these armies become engaged with one another? And more importantly, how do we know who's won the battle?
Simply put, both armies, North and South will organize their men into groups of approximately 350 called regimens. These men will stand shoulder to shoulder in two lines called ranks, and March toward the enemy. Once they are in range, they will fire their weapons at one another or charge each other with the bandit.
When one side is driven the other away, the battle has been won. We call this method linear tactics. Now remember that in those times, the way that soldiers fight is dependent on the weapons they use. And during the Civil War, both soldiers of North and Southern armies are going to use a rifled musket, which is a single shot weapon that has an effective range of about 400 yards, and it can be fired roughly three times in a minute.
One man with one of these weapons won't be very effective, but three or 500 or a thousand men is a different story. This is why one regimen on a battlefield can make a difference.
Even in larger, more complicated battles, the concept remains the same, maneuver your line of battle into position where it can damage the enemy with its firepower or subdue it in hand to hand combat.