World War I: Brazil declares war on the Central Powers.

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.

During World War I (1914–1918), Brazil initially adopted a neutral position in accordance with the Hague Convention as an attempt to maintain markets for its export products, mainly coffee, latex, and industrially manufactured items.