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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 an army's strength? Or did an army on a forced march lose significantly LESS than one-third its strength because it sloughed off (by definition) its weakest members?

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The strategic advantages may rule the day here coupled with good or available supply at the end of the march. If the forced march gives one side an major advantage in either then it is worth doing. Otherwise, not so much. Ultimately, it is all about winning the war not a battle. –  Sardathrion Nov 16 '11 at 17:07
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The dynamics of a forced march: management cuts the deadline, programmers work overtime and on weekends, people quit and the product delivered is shoddy. Oh wait, wrong SE site... –  quant_dev Nov 19 '11 at 17:42
    
@quant_dev: Not a bad commentary. Sun Tzu would have agreed with you. –  Tom Au Nov 19 '11 at 17:48
    
Let's just say that I had to vent somewhere my distaste for the overuse of military rhetoric in business. –  quant_dev Nov 19 '11 at 18:37
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@quant_dev - that's a Death March, not a forced march. –  Oldcat Jun 12 at 23:07

4 Answers 4

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.

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Historically, 16th through 19th centuries, a forced march was not an extra speed march, but rather a double march; a second march during the heat of the day, as follow up of the morning march from sunrise to mid-day. It had to be used sparingly because the men would outmarch their supplies and carry their packs through the hot afternoon sun, risking heatstroke and dehydration. –  Pieter Geerkens Jun 13 at 16:08

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.

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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.

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A forced march isn't necessarily a "last" resort, but it is certainly an "extreme" resort! –  Steven Drennon Nov 18 '11 at 15:44
    
Through the 18th century, until the French Revolution, a forced march was the only means to compel the opponent to battle. This is a large reason why the Frederickian era is so dominated by sieges rather than field battles. –  Pieter Geerkens Jun 13 at 16:11
    
The Green - Cornwallis would be a retirement and a pursuit rather than a forced march per se, which usually is voluntary. –  Oldcat Jun 15 at 18:20

It is foolish to apply literally the maxims applicable to a mostly-levy army 2500 years ago to more modern forces. There is much of value to be found in The Art of War, as well as much that has faded from applicability over the centuries; one must read with understanding to distinguish one from the other.

Caesar, only 400 or so years subsequent to Sun Tzu, routinely force-marched his veteran legions during the campaigns in Gaul to great effect. The massively efficient organization that was a Roman Legion on campaign, financed by the tributes and looting of an Empire, rarely had difficulty keeping Caesar's legions supplied during these maneuvers, which left the Gauls constantly wondering when and where Caesar's forces would appear next. This ability to surprise the Gauls, and apply superior force consecutively to a series of Gallic forces, were at the core of Caesar's success. Note that several of Caesar's Gallic campaigns were initiated in spring and even winter, when his troops would not have to force march through the heat of a summer afternoon.

Frederick likewise applied the same tactics 1700 years later, again to great effect. Force marching onto the supply lines of one's opponent was the only means (other than relieving a siege) of forcing battle in that era of massive supply trains and linear tactics, and Frederick used this to pick and choose the location and timing of his battles carefully.

In April 1809 Oudinot force-marched his entire 2nd Corps of (mostly) new recruits for 5 days to play a key role in the destruction of Charles's left wing under Hiller. Note that Davout had been stockpiling foodstuffs and other supplies around central Bavaria for 4-6 weeks before Charles invaded, so outrunning supplies was not an issue for Oudinot. Although Oudinot sustained greater then usual attrition from this march, it was more like 10% over the five days than 30. See the excellent descriptions of the early 1809 Campaign in Saski: Campagne de 1809 en Allegmagne et en Autriche and in Jack Gill: Thunder on the Danube Volume 1.

During the era of Marlborough, Frederick and Naploleon the normal daily routine of an army on campaign was to:

  • arise an hour before sunrise, have breakfast, and break camp;
  • march for 6-7 hours until mid-day;
  • possibly battle in the afternoon for 3-4 hours; and finally
  • make camp; forage for additional foodstuffs; maintain equipment; enjoy dinner; and bed down for the night.

If no battle was being fought the men would often siesta for 2 hours after the (light) mid-day meal, as the overnight sleep allowance was typically only 5.5 to 6 hours.

Much of this routine was dictated by the fact that the infantry marched at nearly double the pace at which the supply trains could proceed, bearing in mind that the daily set-up and take-down of kitchens for 10,000 troops (typical division size) was a lengthy operation in and of itself, after which food for the same number of troops had to be prepared. Much of the cooking could occur overnight in order to supply each soldier with his daily (loaf of) bread, but the rest had to take place while there was daylight to see by.

Note also that horses must spend several hours a day eating in order to stay fresh due to the low caloric density of pasture. Oats and other higher-density foodstuffs can be used as an occasional top-up, but will make the horses sick if over supplied.

When a body of men went on a force-march it would forego the afternoon battle and foraging in order to make a second march of 6 or so hours through the heat of the afternoon, under full pack and with unreliable water supply. This would be facilitated when the troops were trained veterans; foodstuffs and water were reliably available; and the weather was accommodating (as in April 1809).

The additional exertion would always lead to a higher rate of breakdown for mean, horses and equipment; but for trained and well-supplied forces this was not overwhelming. Of much greater import to a commander was to minimize, over the entire length of a campaign, the amount of time that his force had to march through the heat of the hot summer afternoons, rather than to worry about a couple of days here and there of forced marching.

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