By 1867, Vietnamese emperor Tu Duc (1829-83) had conceded all of Cochin China (southern Vietnam) to the French, who were looking for a trade route into southwestern China. French explorer Francois Garnier (1839-73) was sent with a small force to Hanoi to settle a dispute between local officials and a captured French trader-smuggler. Hanoi's refusal to capitulate to Garnier, who sided with the French trader, led to the storming and seizure of the Hanoi fortress; Garnier went on to seize other forts in the Red River delta area and extended his control over most major Vietnamese cities in the north. A joint Vietnamese-Chinese force, called the "Black Flag Pirates," opposed Garnier and killed him in a battle late in 1873. French ships were captured, and pro-French Christian towns were torched in Tonkin (north Vietnam), which the French decided was not worth trying to control and temporarily abandoned the effort, withdrawing from Hanoi and other cities. Tu Duc was impelled to recognize that the French hold over Cochin China was indisputable and the Christian missionaries and converts were free from persecution (his promise); the French gained use of the Red River as a trading lane.