Upon returning to Vietnam in 1861, Tộ briefly served the French colonial forces in southern Vietnam, translating Chinese language documents for the French navy's Admiral Charner in the negotiations that eventually led to the "unequal treaty" signed by France and Vietnam in 1862. This came in the wake of a 1859 invasion by France and Spain of Vietnam. The Europeans had intended to attack Đà Nẵng and Huế in central Vietnam and to force Tự Đức to surrender, so that they could extract territorial and trade concessions. Since the central areas near the imperial fortress were heavily defended, the European forces diverted themselves to the southern region, and in 1862, the Nguyễn court ceded three southern provinces to the French. In addition, the free practice and propagation of Christianity was allowed, in addition to financial compensation.