What Are the Dynamics of A "Forced March?"

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.

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 men, 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.

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.