BELLEAU, France — It was the spring of 1918, and the German army was making a final push toward Paris. The only obstacle in their way was a contingent of Allied troops, including untested US forces near the Marne River in northern France.

Among them was US Army First Lieutenant Gordon Kaemmerling, a Harvard graduate from Erie, Pa., who had jumped at the opportunity to help the United States leave its shell of isolationism and join the war.

On June 6, 1918, US forces attacked, storming across the open fields near Belleau Wood. Germans sprayed them with shells and machine-gun fire from a densely forested hill. Without proper artillery cover, the Americans were mowed down easily at first.

In the chaos, the 26-year-old Kaemmerling rushed to help his comrades and was cut down by shrapnel and bullets.

The bravery of Kaemmerling and others helped the US troops chase the German forces out of Belleau Wood by the end of the month. The battle became a defining moment in World War I, not just containing the German push along the Western Front but proving the US military mettle for all to see.

Victory bonded the Allies, and that friendship became the cornerstone of global diplomacy for most of the last 100 years.

Even amid some trans-Atlantic strains, that partnership is being celebrated during the centennial of the battle of Belleau Wood as the United States marks the Memorial Day holiday.

Germany acknowledged that the arrival of the US forces on the Western Front would be a burden, but peace with the Bolsheviks in Soviet Russia meant that German troops could reinforce another onslaught on France.

After more than four years of fighting, victory seemed possible for Germany.

The American force ‘‘was still considered a very untried organization, and among the French and British, they were not sure how well they would perform,’’ said retired US Army Colonel David S. Jones, a historian.

The initial plan was to give many of the US forces more time to train before being thrown into battle, but Germany had other ideas.

In desperation, the French asked US General John J. ‘‘Black Jack’’ Pershing for the immediate deployment of his some of his troops to stop the gap near the Marne, northeast of Paris. US soldiers and Marines were sent to Belleau.

Their gung-ho enthusiasm became the stuff of legend. The battle is one of the first lessons taught to any Marine, said Owen Gardner Finnegan, a Marine who served in Afghanistan.

Finnegan recently visited the Ainse-Marne American Cemetery in Belleau Wood, where Kaemmerling is one of 2,289 Americans laid to rest.

Because of their ferocity, the Americans ‘‘stopped the most advanced army in the world at the time,’’ Finnegan said.

Marine Corps lore has it that one officer, told there was a general retreat, said, ‘‘Retreat? Hell, we just got here!’’

The Americans made their vigor, youth, and resilience count against the more experienced but battle-weary Germans, who were approaching their fifth year of fighting.

When the Americans were confronted with ‘‘intense enemy fire, instead of ducking, instead of retreating, they charged,’’ said French historian Jean-Michel Steg.

Steadily they established themselves in hostile terrain where any tree could hide an enemy. They stood their ground in man-to-man fighting.

Instead of marching on Paris, the Germans soon found themselves in retreat.

Much more was at stake than a patch of ground along a 350-mile front line. “It became a test of will,’’ Steg said.

Bolstered by the Americans, the Allies were pushing the Germans back. By the end of 1918, more than 2 million Americans were on the front.

Belleau Wood ‘‘definitely was a critical point in the road for America becoming a world power,’’ Jones said.

Other successes by US, French, and British Commonwealth forces led to the armistice of Nov. 11, 1918.

On a spring visit to Washington, French President Emmanuel Macron brought a sapling from Belleau Wood to the White House for a replanting ceremony with President Trump.

Macron said the tree could take root ‘‘as a symbol of the sacrifice and the common battles that France and the United States have led together.’’