During which conflict were the Marines called "Teufelhunden" by German soldiers?

The term "Teufelhunden," which translates to "Devil Dogs," was used by German soldiers during World War I to describe U.S. Marines. This nickname originated during the fierce fighting at the Battle of Belleau Wood in 1918, where Marines demonstrated exceptional bravery and tenacity against the German forces. The German soldiers were impressed—and somewhat intimidated—by the ferocity and fighting spirit of the Marines, which led them to use this moniker. This term has since been embraced by the Marine Corps and remains a point of pride in their history.

While the other conflicts listed—World War II, the Korean War, and the Vietnam War—were significant in their own rights, the specific attribution of "Teufelhunden" distinctly points to the events and recognition that arose during World War I.