Vietnamese guerrilla leader Trương Định chose to fight on against European forces, defying Emperor Tự Đức and the Treaty of Saigon.

Trương Định, sometimes known as Trương Công Định, was a mandarin (scholar-official) in the Nguyễn dynasty of Vietnam under Emperor Tự Đức. He is best known for leading a guerrilla army in southern Vietnam against French forces in defiance of the emperor. He refused to recognise the 1862 Treaty of Saigon that ceded Vietnamese territory to France.

Tự Đức was the fourth emperor of the Nguyễn dynasty of Vietnam, and the country's last pre-colonial monarch. Ruling for about 36 years from 1847 to 1883, this made him the longest reigning Nguyễn emperor.

The Treaty of Saigon was signed on 5 June 1862 between representatives of the colonial powers, France and Spain, and the last precolonial emperor of the Nguyễn dynasty, Emperor Tự Đức after the coalition's invasion during the Cochinchina campaign. The signatories were Louis Adolphe Bonard (France), Carlos Palanca Gutiérrez (Spain) and Phan Thanh Giản (Vietnam). Based on the terms of the accord, Tự Đức ceded Saigon, the island of Poulo Condor and three southern provinces of what was to become known as Cochinchina to the French. The treaty was confirmed by the Treaty of Huế signed on 14 April 1863.