Oman, nothing if not a confident 24-year-old, had a message for his elders in his essay, although they were too obtuse to learn it until the events on the battlefields of Flanders 30 years later demonstrated how right he was. But there is more to it even than that, because the same lesson — suitably modified for modern times — applies today, and the masters of war of our time are proving as obtuse and stupid as the generals and politicians of late Victorian and Edwardian time.

While the theme of the essay is tactics, the lesson concerns the difficulty of recognizing when the terms of battle have fundamentally changed.

In brief, from the Battle of Adrianople (378), the supremacy of the Roman infantry legion was superseded by the charge of the heavy armored horseman — the cataphract, a development of, primarily, Iranians that spread to dominate Europe and western Asia for over a thousand years, fundamentally reshaping economies, politics and social organization.

From the late 13th century, two innovations began to overthrow the undisciplined, aristocratic knights: the phalanx of Swiss pikemen and the corps of Welsh longbowmen. Yet for over a century, the knights refused to recognize the change, no matter how many of them were slaughtered at, for example, Crecy.

The run of the Swiss and the English was much shorter, less than two centuries, and they, too, were very late in recognizing that a new way of fighting had made them vulnerable.

The introduction of firearms set up a period of innovation and confusion so that for some time there was no obvious best form of fighting, but the introduction of the long-range rifled musket in the 1840s began a new period of mastery.

The generals did not know it, as proven by Grant at Cold Harbor in 1864, and when Oman wrote in 1884, the supremacy of infantry in field works armed with long range weapons was still denied. The supremacy was enhanced by the introduction of breechloaders, repeaters and finally of machine guns. Small armies could defeat big ones, as the Turks demonstrated at Plevna.

The generals, who tend always toward incompetence, did not notice, until July 1 on the Somme in 1916 when more men were killed in a day than had happened since, perhaps, Cannae 2,100 years earlier.

The tank was invented to overcome the fieldworks, but its run was short. It was over for most conflicts by 1945.

For the past 70 years, in most conflicts where one side had tanks and planes and the other did not, the tankless, planeless fighters prevailed. As long as the population shelters him the guerrilla — if he can get submachine guns, rocket grenades and bullets, as he usually could in the age of nuclear standoff between the great powers — prevails.

The United States and the NATO nations spend close to a trillion dollars a year on their militaries. More planes, more ships, more radars — however necessary to deter similar national actors — are unlikely to gain results against committed fighters who have the backing of locals.