U.S. troops in tanks trundle toward the battle line in the Forest of Argonne, France, Sept. 26, 1918. Atlas Archive/The Image Works

Half a century ago, on the 50th anniversary of World War I, the diplomatic historian William L. Langer described the war as “the wellspring of our discontents.” It remains that now, as we mark its 100th anniversary: Many of today’s most virulent struggles, from Ukraine versus Russia to the Middle East and its borders, have their origins in the Great War and its outcome. Indeed, the Islamic State boasts that its self-declared new “caliphate” is erasing the boundary between Syria and Iraq set by the World War I peace settlement.

A less noticed consequence of World War I is also still with us: the transformation of oil into a strategic commodity, central to international politics and critical to the fate of nations.

For the first half century after the discovery of oil in Pennsylvania in 1859, the oil industry was in the business of illumination—providing kerosene for the lamps that pushed back the night in urban and rural areas. John D. Rockefeller became the world’s richest man as a merchant of light.

Only at the beginning of the 20th century did automobiles—powered by gasoline, which had been a virtual waste product—start to take to the roads. But their numbers were still tiny. In the years just before 1914, the world’s major navies began to shift toward replacing coal with oil in hopes of gaining speed and maneuverability and cutting refueling time.

Winston Churchill, as first lord of the Admiralty, was the great champion of the Royal Navy’s switch to oil, proclaiming, “Mastery itself was the prize of the venture.” But in the years running up to the war, he encountered great opposition from both the Royal Navy and Parliament. Relying on oil, traditionalists argued, would leave the Royal Navy dependent not on its traditional, reliable supplies of Welsh coal but on oil from a highly unstable and unreliable nation: Persia (now Iran). In response, Churchill declared what would become the enduring principle of energy security: diversification. “Safety and certainty in oil lie,” he said, “in variety and variety alone.”

But when war broke out in August 1914, oil hardly figured in the planning of the European powers. Their focus was on horses. The rule of thumb was one horse for every three soldiers. That in turn imposed a huge logistical burden: Each horse would require 10 times as much food as each soldier.

Horses had obvious limits. Wars in the second half of the 19th century—the U.S. Civil War, the Franco-Prussian War—depended on railroads to move troops quickly. But rail was inflexible: Trains could run only where tracks did. The newly invented internal combustion engine, running on oil products, provided much more flexibility.

That value was proved in the very first months of the war, when the German army rapidly advanced almost to the doorstep of Paris. The French government evacuated the capital, and it seemed likely soon to sue for peace. The military governor of Paris, Gen. Joseph Gallieni, violently disagreed. He was convinced that the Germans could be stopped, but with the railway system disrupted, there seemed no way to move enough troops. In a brilliant act of improvisation, Gallieni commandeered all of Paris’s taxi cabs to rush French troops to the front—and halted the German advance. Motorized warfare had arrived.

Oil and the internal combustion engine quickly transformed the battlefield. The British Expeditionary Force that landed in France in August 1914 had a mere 827 motorcars and just 15 motorcycles. By war’s end in 1918, British military vehicles included some 56,000 trucks, 23,000 motorcars and 34,000 motorcycles and motor bicycles. When the U.S. entered the war in 1917, it brought another 50,000 vehicles to the European front.

All of this enabled the Allies to move troops and supplies swiftly. After the war, some described the Allied victory as the triumph of their trucks over Germany’s locomotives.

But it wasn’t only the truck that proved transformative. Militaries looked frantically for a response to trench warfare and the machine gun, which made crossing the battlefield lethal. The answer was the oil-powered armored vehicle, based on a new contraption that had just appeared in the U.S.: the agricultural tractor. Developed under various code names, including the “cistern” and the “reservoir,” it eventually became known as the “tank.” When the German High Command declared in October 1918 that victory was no longer possible, the first reason it gave was the introduction of the tank onto the battlefield.

Oil-powered vehicles proved revolutionary above the battlefield too. Before the war, a top French general had dismissed the airplane as “good sport, but for the Army the aeroplane is worthless.” As late as the start of 1915, the British aviation industry had been able to build just some 250 planes. But innovation came quickly; by that summer, every plane that had been in the air at the beginning of the war a year earlier was considered obsolete. Over the course of the war, Britain produced some 55,000 planes; Germany made 48,000.

Airplanes were first used for reconnaissance, then for aerial combat and bombing—with dramatic impact. The speed of new aircraft doubled to 120 miles an hour, and some operated at a height of up to 27,000 feet. What the chief of the British Air Staff said of the Royal Air Force also applied to military aviation in general: “The necessities of war created it in a night.” And of course, all their engines ran on oil.

But nothing dramatized the new centrality of oil so much as the German submarine attacks on U.S. shipping, particularly tankers carrying oil to Europe from the U.S. (then by far the largest source of oil). As tanker sinkings mounted, the Royal Navy even considered giving up oil and going back to coal. “The Germans are succeeding,” warned the U.S. ambassador in London. “They have lately sunk so many fuel oil ships” that Britain was “in a perilous situation.” By coordinating shipping supplies and instituting convoys, the Allies managed to stave off the U-boat threat and keep the oil flowing across the Atlantic—but only just in time.

Within a few short, brutal years, a war that had begun with expectations of cavalry charges ended with worries about oil supplies. Such was the transformation that, just days after the Armistice, Lord Curzon, who was about to become Britain’s foreign secretary, declared, “The Allied cause had floated to victory upon a wave of oil.”

Oil had become the strategic commodity, essential for the exercise of power. It remains that today—as well as a “wellspring of our discontents.”

—Mr. Yergin is the author of “The Prize: the Epic Quest for Oil, Money, and Power” and “The Quest: Energy, Security, and the Remaking of the Modern World.” He is vice chairman of IHS, a research and consulting firm.