World War I: U.S. Marine Corps suffers its worst single day's casualties during the Battle of Belleau Wood while attempting to recapture the wood at Château-Thierry (the losses are exceeded at the Battle of Tarawa in November 1943).

World War I or the First World War, also known as the Great War, was a global conflict between two coalitions: the Allies and the Central Powers. Main areas of conflict included Europe and the Middle East, as well as parts of Africa and the Asia-Pacific. There were important developments in weaponry including tanks, aircraft, artillery, machine guns, and chemical weapons. One of the deadliest conflicts in history, it resulted in an estimated 30 million military casualties, plus another 8 million civilian deaths from war-related causes and genocide. The movement of large numbers of people was a major factor in the deadly Spanish flu pandemic.

The United States Marine Corps (USMC), also referred to as the United States Marines or simply the Marines, is the maritime land force service branch of the United States Department of Defense. It is responsible for conducting expeditionary and amphibious operations through combined arms, implementing its own infantry, artillery, aerial, and special operations forces. The U.S. Marine Corps is one of the six armed forces of the United States and one of the eight uniformed services of the United States.

The Battle of Belleau Wood was a major battle that occurred during the German spring offensive in World War I, near the Marne River in France. The battle was fought by the U.S. 2nd and 3rd Divisions along with French and British forces against an assortment of German units including elements from the 237th, 10th, 197th, 87th, and 28th Divisions. The battle has become a key component of United States Marine Corps history.

Château-Thierry is a French commune situated in the department of the Aisne, in the administrative region of Hauts-de-France, and in the historic Province of Champagne.