What Are the Dynamics of A “Forced March?”

Question

In the "Art of War," Sun Tzu opined that if you put your army on a forced march of a certain speed, you will lose one-third of your troops along the way. A rough rule of thumb was that an army would start to disintegrate after losing about one third of its strength.

At what point do armies tend to break?

Were the advantages to be gained by a forced march usually enough to compensate for such a diminution of a army's strength? Or did an army on a forced march lose significantly LESS than one-third its strength because it sluffed off (by definition) its weakest members?

Answer 1

I believe Sun Tzu's comments were meant to be more cautionary than anything. A commander has to consider the consequences of such an action in order to make a more informed decision. If you need to conduct a forced march to obtain a strategic advantage, such as cutting off the enemy supply line or gaining control of a water crossing before they can, then you need to be aware that you will not be at full operating strength once you get there. If the objective can't be obtained with less than your full strength, then you may have to decide another course of action.

Quite simply, a forced march can be applied to any march during which the troops are expected to move at a faster pace than normal. In the Marines, they gave us plenty of opportunities to "practice"! This was one of our lessons in Officer Candidate School, and by witnessing it first hand in a training exercise, it gave you a better appreciation for the limitations of your troops if you decided to take such action.

Answer 2

From what I now understand, a "forced march" was a "last resort: move, something akin to "sacrificing a limb to save a life."

The reason it was so dangerous was because it was "guaranteed" (according to the observation of Sun Tzu) to lose part of your army (if conducted at a certain speed). To risk your army in this way, one must be seeking an advantage of a comparable magnitude. Such an advantage (if achieved) would be one that would win the war (or at least the campaign) by itself.

The consolation was that losing "one third of your men" was probably NOT tantamount to losing one third of your strength, because you were losing your one-third WEAKEST men (as opposed to losing one third at random, or worse, your bravest one third).

During the American Revolution, America's General Nathaniel Greene successfully led Cornwallis one a forced march through the Carolinas. Greene lost about half his 4,000 men this way (mostly militia), while Cornwallis lost one-third of his 3,000 men (more evenly distributed). The result was two thousand veterans on each side that met at the (basically drawn) battle of Guildford Courthouse.

Answer 3

The rule of thumb that an army would start to disintegrate after losing about one third of its strength applies during the shock and stress of battle. (This is particularly important as such disintegration is what causes many battles to be so decisive; two similar size armies could slog it out for hours, suffering gradual losses, but when one army finally breaks it usually lost a much larger number of men.)

And Sun Tzu's estimate would vary greatly depending on the quality of the army and its current coherence. Highly motivated armies could lose very little, while poorly motivated ones could lose more.

Forced marching was as likely to reduce the battle effectiveness of an army as it was to actually reduce the army's numbers. This would be through fatigue, stragglers or similar, all of which could be recovered with sufficient rest.

So, if the aims of the march could be achieved at the reduced effectiveness (say, capturing a key defensive point before the enemy), it may be possible for the forced march to have little long term impact.

An example of both the advantages and disadvantages of forced marching can be seen in the example of Harold Godwinson's campaign in England in 1066. Facing off against a possible invasion by the Normans along the south coast, he was informed of the invasion and capture of York by Harald Hardradr of Norway.

He marched his bodyguard (highly motivated & disciplined!) north and caught the Norse away from camp without their armour, and defeated them soundly. Harald Hardradr was killed with an arrow to the throat.

He then marched them south again, to find William (soon to become 'the Conqueror') ashore, and stood against him at Hastings. Harold's loss to the Normans is at least partly attributed to the fatigue of the two forced marches his men had had to endure.

A couple of notes here: Harold chose to march with only his best troops to maximise speed and minimise losses; and the value of surprise against the Norse outweighed any disadvantages from the march.