This photograph is on display at the Hoa Lo Prison (Hanoi Hilton) in the exhibits section and shows those who were executed in the Ha Thanh or Hanoi Citadel (Vietnamese:Hà Thành đầu độc) poisoning plot which occurred in 1908 when a group of Vietnamese tirailleurs attempted to poison the entire French colonial army's garrison in the Citadel of Hanoi. According to the rebel plan, Vietnamese cooks were to neutralize the French garrison by poisoning their dinner foods while Vietnamese soldiers (Grade Indochinois) would attack and prevent French reinforcements from coming to the aid of the Hanoi Citadel.
On the evening of June 27, 1908, a group of Vietnamese cooks managed to mix datura poison with the food they had prepared for a dinner party. The poison immediately knocked out nearly 200 French troops but it did not kill them. However, one of the cooks later felt guilty and went to church for confession and the French priest reported the confession to government officials. The French general-in-charge in Hanoi immediately proclaimed martial law and ordered the arrest of plot leaders and plotters while Hoang Hoa Tham, waiting outside of the Citadel, realized the plot had failed and withdrew all of his troops.
In response to the plot, the French executed 13 plotters by guillotine and on July 8, 1908, another 24 death sentences were announced while the remaining rebels were sentenced to life or exiled.
As for Hoang Hoa Tham, the French authorities began to hunt him down and they attacked and defeated him in 11 major battles from January 29 through November 11, 1909 before finally surrounding him in Yen The. However, he still managed to flee and kept on fighting until he was assassinated in 1913.