“The Great War” and “The war to end all war” proved to be neither. The conflict that would bleed to death some of Earth’s proudest empires — while killing 16.5 million soldiers and civilians — later would be christened World War I to distinguish it from the next, even deadlier slaughter.

WWI also created a precedent and an image: If you wonder why Europe today relies on the U.S. and not, say, Brazil or China to help protect Ukraine, the answer traces back a century, to one battle in which American doughboys helped repel Germans grinding toward Paris.

One hundred years ago this week, the interlocking machinery of European alliances creaked into action: Austria-Hungary declared war against Serbia on July 28; Russia mobilized to defend the Serbs; Austria’s ally, German Kaiser Wilhelm Hohenzollern, declared war against his cousin, Russian Czar Nicholas Romanov II; . War would ensnare France, Britain, the Ottoman Empire and eventually the U.S.

On Aug. 2, 1914, the Tribune editorial board wrote a piece titled, “The twilight of the kings.” It is trenchant and lyrical, foreshadowing not only the war’s “feudal slaughter” but also sclerotic monarchies, after centuries of family rule, finally yielding to democracies: “The republic marches east in Europe.” That editorial’s false prediction: “Western Europe of the people may be caught in this debacle, but never again.”

“Twilight” reflected an America reluctant to join another continent’s fight. Woodrow Wilson was re-elected president in 1916 because “he kept us out of war.” By April 1917, he no longer could. Submarine warfare had torpedoed U.S. neutrality.

The rest of America’s 20th century — and global expectations of this country in the 21st — flowed straight from the languid waters of a river in France.

In summer 1918, at the Second Battle of the Marne, forces of France, the U.S., the United Kingdom and Italy repulsed a strong German offensive. The Americans, fresher than their war-exhausted comrades, tipped the battle that tipped the war: Tribune correspondent Floyd Gibbons wrote, “I never saw men charge to their death with finer spirit.” After the Allies secured victory on Aug. 6, their muscular counteroffensive took only 97 days to break the German Empire.

Footnote to history: Most of what Americans learned early on about the Second Battle of the Marne came from Gibbons’ only weapon, his notebook. He had wangled his way to the front, where reporters weren’t permitted. As he crawled to aid a wounded major, the third German bullet to hit him took out his left eye. For his heroism, France decorated civilian Gibbons with its military Croix de Guerre (Cross of War) with Palm.

For Americans, the war lasted only 16 months and didn’t scar their soil or their cities. And while any casualty causes suffering, the U.S. death toll of 117,000 was less than 1 percent of the total carnage.

The U.S. was more than Europe’s military rescuer. The old continental notion of invincible monarchies — of noble empires as the world’s organizing principles — lay in ruin. And if the war itself hadn’t wrought enough human havoc, the mobility of armies during and after it helped spread an influenza pandemic that killed 50 million to 100 million people — perhaps 5 percent of Earth’s human population.

As all of this settled, one nation stood strongest:

The war effort had made America a greater industrial, financial, naval and diplomatic power. Herbert Hoover’s American Relief Administration, which provided food to millions of starving Europeans in 23 nations, reinforced Wilson’s dictum when the U.S. entered the war: This country wanted no tangible gain from its outcome. Wilson instead sought to “make the world safe for democracy.”

Instead of territorial gain, the U.S. had earned a vast reputational gain. In the eyes of the world, the formerly isolationist Americans were strong, resolute, and willing to pay their own blood and treasure to buy the freedom of others.

So when subsequent menacing “isms” — fascism and communism — threatened Europe, Europeans turned again to the Americans.

The U.S. most notably answered that call in the war for which this year marks another anniversary — World War II started 75 years ago, in 1939.

In succeeding decades, with U.S. involvement in the growth of the North Atlantic Treaty Organization and in a long list of geopolitical crises, America’s service to Europe has exacted high costs. What was once the most self-reliant continent now sniffs smoke and looks west, trusting that if fire erupts, America will respond.

This page often criticizes affluent European countries for refusing to spend more on their military preparedness. The collapse of the Soviet Union gave Europe essentially the same sense of calm that had made a massive war unthinkable before World War I. War historian Margaret MacMillan writes in the Wall Street Journal of how that war brutalized Europeans who “had grown accustomed during the largely peaceful 19th century to think that peace was the normal state of affairs.”

Today, the guns of August that savaged poppy-dotted fields in 1914 echo faintly in wheat fields strewn with airliner wreckage and body parts. Crisis in Ukraine has startled those in the West who had come to see Russia as subdued, tamed.

Again, a Europe unable and unwilling to resolve its disputes turns to Washington, where politicians are unable and unwilling to agree on how aggressively America should lead.

The above editorial appeared in the Chicago Tribune on July 27.