The French had not come to Vietnam intending to build an empire. The Cochinchina campaign began in 1858 as a limited punitive expedition, a show of force to punish the Vietnamese court for persecuting Catholic missionaries and to extract trade concessions. France and Spain sent a joint force that captured the port of Tourane and then Saigon, taken on February 17, 1859, under Admiral Charles Rigault de Genouilly. But the Vietnamese did not capitulate. Instead, they besieged the Franco-Spanish garrison in Saigon, pinning the European forces inside the city while a massive Vietnamese army dug in around them. For nearly two years the situation held in a grinding stalemate, the French too few to break out, the Vietnamese too cautious to storm the defenses. What changed everything was the end of the Second Opium War in China, which freed up French troops for redeployment southward.

The reinforcements that arrived in early 1861 constituted the largest French naval force Vietnamese waters had ever seen. Admiral Leonard Charner commanded a fleet of 70 ships carrying 3,500 soldiers under General Elie de Vassoigne. The squadron included the steam frigates Imperatrice Eugenie and Renommee, the corvettes Primauguet, Laplace, and Du Chayla, eleven screw-driven dispatch vessels, five first-class gunboats, seventeen transports, a hospital ship, and half a dozen armed lorchas purchased in Macao. It was an overwhelming display of industrial-age naval power directed against a pre-industrial army. Charner's fleet would not be surpassed in these waters until the creation of the French Far East Squadron more than two decades later, on the eve of the Sino-French War.

The entrenched camp at Ky Hoa was a rectangle roughly 3,000 meters by 900 meters, divided into five compartments by internal traverses and armed with more than 150 cannon. The siege lines stretched 12 kilometers. On the morning of February 24, French artillery advanced to within a kilometer of the Vietnamese positions and opened a bombardment. Infantry formed up in battalion columns behind the guns. Then the mountain artillery pushed forward to within 500 meters of the Redoubt Fort, and the assault columns split: soldiers to the right under chef de bataillon Allize de Matignicourt, sailors to the left under capitaine de fregate Desvaux. They charged. The fighting was fierce enough to wound both the French commander de Vassoigne and the Spanish commander Colonel Palanca y Guttierez. By the end of February 25, the Ky Hoa lines had fallen.

French and Spanish casualties were 12 dead and 225 wounded, numbers the French themselves called relatively heavy. Many of the wounded, including Lieutenant-Colonel Testard of the marine infantry, later died. The Vietnamese losses were far worse. French estimates placed their dead at around 1,000. General Nguyen Tri Phuong was wounded during the battle. Two Vietnamese commanders, Tan-ly Nguyen Duy and Tan-tuong Ton That Tri, were killed in action. Pham The Hien, the deputy of Nguyen Tri Phuong, was gravely ill and died days later. These were not anonymous soldiers. They were among the most experienced military leaders Vietnam possessed, and their loss crippled the country's ability to resist what came next.

With the Ky Hoa lines broken, the French and Spanish shifted from defense to offense. My Tho fell in April 1861. Bien Hoa and Vinh Long were captured in early 1862. Each defeat pushed the Vietnamese court closer to the negotiating table, and by April 1862, Emperor Tu Duc sued for peace. France was in no mood for generosity. What had started as a minor punitive expedition had become a long and bitter war, and the French demanded territorial compensation. Tu Duc was forced to cede the three southernmost provinces of Vietnam, Bien Hoa, Gia Dinh, and Dinh Tuong, to France. The colony of Cochinchina was born, with its capital at Saigon. It would endure for nearly a century, shaping Vietnamese society, politics, and resistance movements in ways that would echo through the wars of the twentieth century. The battle at Ky Hoa, fought over two February days in the mud outside a besieged city, was the hinge on which all of it turned.