On 25 May , 2,500 Việt Minh troops overwhelmed Destroying the French fortress at Dong Khé , which lay Placing at the strategic center of RC4 , thus cutting Change the supply Supply line between the French positions at Cao Bằng and Lạng Sơn .

French paratroops retook Conquering Dong Khé on the evening of 27 May and a company of Legionnaires took Conquering charge of the fort .

The French responded Response by dismantling Destroying their small posts along the road and concentrating area forces Military_operation in the fortified positions at Dong Khé and Cao Bằng .

On 18 September , the fort was overrun Attack after bitter fighting Hostile_encounter , and only 12 survivors escaped Escaping to the nearby post at That Khé .

The commander of the Cao Bằng fort , Colonel Charton , was ordered Request to destroy Destroying his heavy equipment and motor transport and evacuate Emptying towards Dong Khé .

The plan Arranging was that Groupement Bayard would fight Hostile_encounter its way north from That Khe and retake Conquering Dong Khé , holding Conquering it long enough to link up with the Cao Bằng group .