On 28 May 1754, Jumonville was sent to warn the Virginian militia leader George Washington against encroaching further into French territory. Alerted to Jumonville's presence, Washington's militia and their native Mingo allies ambushed the French officer's force in the Battle of Jumonville Glen.
The French held Washington responsible for Jumonville's death, calling the ambush an act of war. Once Jumonville's brother, Louis Coulon de Villiers, heard of his death, he and a detachment of 500 men marched towards Washington's militia. The French and Washington's militia eventually fought at the Battle of Fort Necessity.
Washington's militia soon surrendered. In the surrender document, Coulon de Villiers inserted a clause referring to Jumonville's death as an "assassination". Jumonville's death played an important part in sparking the Seven Years' War.