why is an army so vulnerable on its flank?

Factual Questions
1

Watching Ken Burns documentary on the Civil War, I noticed a lot of discussion of one army “flanking” another, i.e. (I think I have this right but corrections welcome) moving around to attack an army on its left or right flank.

I think I understand why this is such an important tactic (or is it a strategy?) but I’m not sure.

I guess the idea is that, if my army is coming head on into your army’s side, then I greatly outnumber you. So, I think that means I can concentrate my fire on a small number of your force, mow them down, and then just keep moving through your whole force “sideways”. Is that right?

2

I asked a similar question a while back. Here is the thread that ensued:

http://boards.straightdope.com/sdmb/showthread.php?threadid=40655

Oddly, some people seemed to think I was using the phrase in its common metaphorical use, despite my clarification in the OP and title.

3

Well it means that the directed fire of the army you are flanking is pointed the wrong way. Not only that, but when they turn to face you massive confusion can ensue.

There’s also the potential to hit heavier, slower units trailing the front and potentially hit supply wagons/vehicles.

4

When you flank a position, you can render the enemy’s cover useless. Hide behind a tree or wall: you may be hidden from someone in front of you, but not to the sides or rear. Lines of retreat and movement are also hampered, because it is now harder to find effective cover. If you are flanked, you also run the risk of being surrounded or having your rear supply formations attacked.

–Patch

5

I like all of the explanations so far, may I help to further visualize:

The old school navy (ships-of-the-line) used to have a similar tactic called “crossing the ‘T’”. They would simply cut in front of the enemy ship at a right angle.

This allows you to bear an entire side battery of guns (30 or 40 guns) on your enemy while he can only use his forward battery (4-8 guns) on you. (in theory there would be a friendly ship following you to attack the same target in a similar way). this way you bring more of your weapons to bear on a smaller force.

It worked well for (initially outgunned) Admiral Nelson at Trafalgar.

6

It’s not really an outnumbering thing.

If you hit the enemy on the flank you’re hitting him where his forces aren’t arrayed for an effective defense. Trenches may run the wrong direction, formations may be pointing the wrong way, overlapping fields of fire may not exist, etc.

A flanked enemy can reposition his lines to respond to a flanking attack, but moving a large military force takes time, particularly if the maneuver is unexpected and carried out in the heat of battle. It’s not enough to just have everyone turn and start shooting in the new direction.

Plus when you flank the enemy you have the opportunity to strike at soft targets behind the main battle line – artillery positions, supply trains, command posts. Destroying or disrupting these support systems can seriously weaken the main army.

Plus there’s the morale hit caused by making the enemy feel like he’s surrounded. If the enemy troops feel like they’re pinned between two attackers they’re more likely to break and run.

7

Usually, in a large formation battle, the force originating an attack has just completed, or is in the simultaneous process of completing a movement to a position of tactical advantage. This may be to a high ground position overlooking the enemy’s expected location, or a position where the enemy must cross water to bring forces to bear. In modern warfare, such geographical features will be large ones, although small forces will still end up trying to get to hilltops, and cross creeks as well.

A flanking maneuver is achieved when one is able to move during or before that attack to a position where that tactical advantage is lessened or eliminated. It often is to one side or “flank” of the opposition forces, but it can even be a maneuver to a stronger position to the rear, which the opposing force must move to engage, exposing themselves to less protected terrain as they move. (Perhaps to forces left in place as the main force retreats.)

The more thoroughly entrenched the position is, the more effective a flanking maneuver can be. If you are dug into bunkers below a ridge line, you have complete free fire across the low ground facing foreword, but very little free fire across the length of your line, or behind you. Forces coming unexpectedly along the ridge line, or over the hill behind you have entirely flanked your position, and may engage your support forces, effectively ignoring your entrenchment, and your artillery. They can also engage only one end of the line at a time, and the remaining forces must leave their fortified position to engage.

Highly mobile forces, such as a tank force, are much more difficult to flank, and the results, although potentially devastating, gain an advantage very briefly. But, since flanked forces are still not able to cooperate as effectively as forces properly arrayed against an opponent who is where you thought he would be, they will be at an extreme disadvantage until they can counter move. If the terrain makes that impossible, your flanking forces can do damage, as they will, until outside forces are able to change the battlefield.

In one more way, information becomes the key to battlefield survival. If you know where it is, you can kill it. So move, and don’t be where you are expected to be.

Tris

8

Tactic. Strategies are higher level (defeating Germany by opening a second front in Africa), and tactics are lower-level (1st army goes here, 2nd army goes there, navy attacks over yonder). According to one former history prof, strategies can generally be described in one sentence.

I suppose that first example is not a great one, as it is kind of a flanking maneuver, but on a much grander scale than one usually thinks of when referring to flanking.

9

Nitpick: While I agree with your definition of and strategic reasons behind the concept of crossing the T, your last statement is 180 degrees off. Nelson did not cross the T at Trafalgar. He approached the French/Spanish line in two columns. In essence, they crossed his T, although it’s clear that he deliberately did caused this to happen.

That was why his strategy was considered radically brilliant. It violated the standard practice. While the lead ships in his two columns took terrible punishment from the concentrated cannon fire from the enemy line, once he cut the line in two places, the British were able to concentrate more strength at the key points of the battle. (An argument could be made that his strategy counted on the better training, experience and leadership of the Royal Navy, and might not have worked otherwise. In essence, he was deliberately giving up a strategic advantage in order to force a battle, where he could bring his other advantages to bear.)

10

Many others have already explained it well. In the case of battle during the Civil War era, even the potential threat of a flank was enough to encourage an army to fall back on a more protected location. Consider two or three lines of defenders, well-entrenched with artillery behind them on slightly higher ground, as was the case during the last day of the battle of Gettysburg. Directly attacking these defenses would be painful as they have cover while you are charging across a field / river / forest / etc. Not only do you have to rout the first, second, third… line of defenders but you have to push all the way to the back to silence the heavy artillery which is chewing holes in your line. At each stage the defenders have the option of falling back to their next entrenchment, and you just have more ground to cover.

The effects of charging a well entrenched line were repeated over an over again, including Sharpsburg, Fredericksburg, Gettysburg, Cold Harbor, etc. These ghastly displays showed the importance of maneuvering and trying to out-flank the enemy.

Get on the side of him however and your enemy will crumple like a length of string. A defensive line may look miles long from the front but it’s only a couple of hundred of feet wide from the side. It’s much easier to roll up like a crepe than plow like a snow bank.

The most impressive tactical victories were the result of a brilliant flanking maneuver followed by a swift yet relatively bloodless rout. There’s nothing like your comrades fleeing down your line and shouting “They flanked us, run for your life” to get you up and following them. This usually resulted in the flanked army pulling out under the cover of darkness, and often in the Union’s case, the replacement of the commanding General.

11

You are thinking of “Lord Howe’s Manoeuvre”.

“Crossing the T” is terminology from the first half of the 20th century (when battleships could fire all their guns to either side but couldn’t fire their forward batteries aft or their aft batteries forward). At the Battle of Jutland it turned out not to work, and the T that ws crossed could simply turn to line breast and bring all their guns to bear.

Regards,
Agback