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 1st BEP waited Temporary_stay at That Khe while a force of French colonial troops , the Moroccan 1st and 11th Tabors , assembled Come_together at Lạng Sơn .