First World War: The "worst 24 hours in Australia's entire history" occurred when Australian forces unsuccessfully attacked German defences at Fromelles, France.

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 Attack at Fromelles (French pronunciation: [fʁɔmɛl] 19–20 July 1916, was a military operation on the Western Front during the First World War. The attack was carried out by British and Australian troops and was subsidiary to the Battle of the Somme. General Headquarters of the British Expeditionary Force had ordered the First Army and Second Army to prepare attacks to support the Fourth Army on the Somme, 50 mi to the south, to exploit any weakening of the German defences opposite. The attack took place 9.9 mi from Lille, between the Fauquissart–Trivelet road and Cordonnerie Farm, an area overlooked from Aubers Ridge to the south. The ground was low-lying and much of the defensive fortification by both sides consisted of building breastworks, rather than trenches.

Fromelles is a commune in the Nord department in northern France. As of 2019 it had a population of 1,041; its inhabitants are called Fromellois. It is located about 16 kilometres (10 mi) to the west of Lille.