It took place Process_start along Route Coloniale 4 ( RC4 , also known as Highway 4 ) , a road used Using to supply Supply the French military base at Cao Bằng .

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 .

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 .

Meanwhile , General Marcel Carpentier , the commander in chief of French Indochina , decided Deciding to evacuate Emptying Cao Bằng .

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 .