These two images might help us visualize the difference between "strategy" and "tactics" in a military context. These two images are from famous events in the career of Napoleon Bonaparte.
On the left is a map of Central and Eastern Europe in 1812, the year Napoleon launched his disastrous invasion of Russia. There are arrows showing the movement of the French and Russian Armies during this campaign.
On the right is a map of the Battle of Austerlitz, probably Napoleon's greatest victory. This is a much smaller scale map, encompassing just a few dozen square miles, showing the specific placement of specific army units in different villages and on different hills and fields.
Now, don't worry about the specific things going on in these two maps. Just try to see that the map on the left, showing all of Eastern Europe, is a strategic map, while the map on the right showing just a few towns and villages, is a tactical map.
This photograph of a German supply train in 1918 will hopefully give you a sense of just how enormous the requirements of feeding and supplying an army can be. This wagon train literally fades off into the horizon, encompassing hundreds if not thousands of large horse drawn wagons. This supply train would probably only be able to feed a small part of the army for a short period of time. Keeping several million bellies full for four years straight involves moving a LOT of food.
Source: A German transport column moving forward along the Albert - Bapaume road, March 1918. © IWM (Q 60474)
Organized Army of Soldiers vs Disorganized Band of Warriors
In this clip from the 2003 HBO miniseries "Rome," we can see a fun example of how organized units of soldiers could often be far more effective in battle than somewhat less organized armies.
Specifically, here we see a unit of Roman Legionaries fighting against a group of Gallic Warriors during the Roman conquest of Gaul under Julius Caesar. Specifically, the Battle of Alesia in 53 bce
While the presentation of Roman battle tactics here is obviously based on a lot of historical guesswork (as this battle occurred more than 2,000 years ago), it is nonetheless a good way to see how a Roman Army may have faced off against an Army of what the Romans would have referred to as "barbarians."
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Let's take a look at some images to help us visualize how "lines" in armies and battles might look over time.
This is a modern drawing of one of the first organized, formal units of army deployment used in human history: the phalanx. Specifically, this is a representation of the kind of phalanx Alexander the Great would have used in the 300s bce during his conquest of the Achaemenid Persian Empire. Basically, soldiers with spears or pikes and big round shields would stand shoulder to shoulder in a long line, usually anywhere from 8-16 men deep. This allowed the phalanx to push forward against an enemy army with a wall of spears and shields, prevented from being pushed back themselves by being braced from behind by the soldiers in the rear.
Here is a more recent example of a line formation, the kind used in the 18th and early 19th century. Specifically, this is a painting of a line of Prussian grenadiers marching shoulder to shoulder towards the enemy, with a few officers in front leading the line and helping to keep the line straight and moving at the same speed. In the days where muskets and other firearms had replaced spears and swords, line formations were usually much longer and much less deep, to allow as many soldiers as possible to blast away in mass volleys.
And here we can see a "line" formation during the First World War. Specifically, this is a unit of German soldiers in a battle in 1914. We can see that the "line" is not nearly as straight and rigid, with individual soldiers finding their own piece of cover, but are nonetheless deployed to all face in the same direction to pour as much gunfire as possible on the enemy.
Here I've drawn a very simplified diagram of two opposing "lines" facing off against each other, with their fronts, rears, wings, and flanks labeled. We can see that which wing or flank we're talking about is always relative to the direction the army is facing. So one sides right wing always faces the other sides left wing, and vice versa.
Turning the Flank
Below are three simplified diagrams showing how an army might try to maneuver one of its wings to the side of an enemy army, hit it in the flank, and then force the enemy army to retreat or risk getting surrounded. This is known as "turning the flank."
Again, these diagrams are very simplified, but they will hopefully give a sense of how this basic maneuver works.
In phase 1, the center and right of the blue army moves forward as the red army moves forward towards them. meanwhile, the blue army sends its left wing, in this case three units strong, to swing around the side of the red army.
In phase 2, the left wing of the blue army manages to fire on the right flank of the red army, inflicting massive casualties and sending the rest fleeing in panic, effectively removing the right wing of the red army from the fight.
In phase 3, the right wing of the red army has all been killed or forced to flee, and now the rest of the red army is in the process of being surrounded. The front and center of the blue army advances forward while the left wing of the blue army, who has managed to get the drop on the red army, moves in to annihilate the rest of the red army's units one by one, which are now being attacked from both the front and the side. The red army is thus forced to retreat, in order to avoid being completely surrounded and destroyed or forced to surrender.
Getting the Enemy to Run
This is the key goal to any battle: force the enemy army to break and run away. Here are two more video examples from film/tv that demonstrate this in action.
The Battle of Waterloo
In this clip from the 1970 film "Waterloo," we see the decisive moment of the Battle of Waterloo of 1815 that sealed the fate of Napoleon Bonaparte: the rout of the Old Guard.
Napoleon's "Old Guard" was his most elite unit of infantry, always kept behind the front lines of a battle and only deployed to break through the enemy towards the end of the battle. At Waterloo, Napoleon sent forward his Old Guard to try to break through the British Army under the Duke of Wellington. But this attack was unsuccessful, and the Old Guard broke and ran in flight. Seeing this most elite and illustrious unit of soldiers routing caused a panic in the rest of the French Army, and soon the whole army was in panicked rout. As Wellington says in this clip "If I ever saw 30,000 men run a race before."
The Battle of Bloody Gulch
This clip comes from the 2001 HBO miniseries "Band of Brothers." In it, we see a much more modern example of a rout of soldiers, specifically during the Second World War as units of German Fallschrimjäger (airborne soldiers) and Waffen SS soldiers are forced to run away from an attack on US Airborne soldiers after being attacked from the side by US tanks.
As we can see, though these soldiers are no longer standing in rigid, shoulder to shoulder formation, the basic rule is the same: one part of the line falters and runs, and soon this panic spreads to the rest of the army.
Charging a Defeated Army with Cavalry
This mosaic from circa 100 bce depicts one of the most famous instances of cavalry charging a defeated enemy army. That of Alexander the Great's Macedonian Companion Cavalry charging the Persian Army of Darius III after the Battle of Issus in 333 bce.
On the left we see a victorious Alexander charging on his horse, and to the right we see the fleeing Persian Army. The most prominent figure on the right, in a chariot slightly above the rest of the subjects, is the defeated King Darius III of Persia, watching helplessly as his army is cut to pieces.
The Cavalry Charge:
When to, and when Not to
Let's look at two more clips from film/tv. Each of these clips will demonstrate an effective use of a cavalry charge on a battlefield, and an ineffective one.
When it Works
This clip, from the 2003 film "The Last Samurai," shows two things really well. First, how a cavalry charge can be incredibly devastating even in an age of firearms. And second, why infantry being charged by cavalry absolutely had to stick together and hold formation.
In this clip, Tom Cruise's character, hired by the Japanese Imperial government in the 1870s to train a new "modern" Japanese Army, is ordered to lead a regiment of these new conscripts to fight a rebel army of traditionalist Samurai warriors.
The Samurai, mounted on horseback, decide to charge the line of Japanese infantry, and Tom Cruise's character orders them to stay in position and hold fire until the Samurai cavalry are very close to ensure the volley does maximum damage. But soon enough these inexperienced troops break and flee, only to be cut down by the charging cavalry. This is a good way to show why staying in formation during a cavalry attack was so vitally important for infantry, especially in the days before rapid-fire rifles and machine guns.
When it Doesn't Work
Here, in another clip from the movie "Waterloo," slightly earlier in the film, we see one of the most famous examples in military history of how not to use cavalry. Long story short, one of Napoleon's sub-commanders orders a mass charge of his heavy cavalry straight at the British lines. The British infantry use a formation known as the "square," which I don't want you to worry about, except to see that it shows that a disciplined unit of infantry sticking together can easily ward off charging cavalry.