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 .

RC4 in the First Indochina War Military_operation .

The area became Becoming a Việt Minh stronghold , except for the RC4 highway .

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 .

Giap then began Process_start harassing Attack French positions along RC4 in northern Vietnam with mines and ambushes .