Operations at Điện Biên Phủ began at 10:35 on 20 November 1953. In "Operation Castor", the French dropped or flew 9,000 troops into the area over three days, as well as a bulldozer to prepare the airstrip. They were landed at three drop zones: "Natasha" (northwest of Điện Biên Phủ), "Octavie" (to the southwest), and "Simone" (to the southeast). The Viet Minh elite 148th Independent Infantry Regiment, headquartered at Điện Biên Phủ, reacted "instantly and effectively". Three of its four battalions, however, were absent.