There is no definitive answer to this question, however there are several possibilities:1. The Section 8 team needed conclusive proof that Col. Styles was behind the drug operation. So far, Sgt. West and Hardy probably only had suspicions.2. Dunbar (Brian Van Holt) might have been sent back to base in order to create a cover story for the disappearance of the other team members.
In order to understand the drug operation in this movie, we must first take a look at the people involved:- Vilmer: Supplied the drugs, named "Combat Cocktails", to soldiers such as Mueller and Kendall. He was also running an operation with a partner, probably Colonel Styles, to smuggle cocaine in military shipments into the United States.- Kendall: Distributed drugs for Vilmer, and was supposedly hired by Col. Styles to kill Sgt. West.- Mueller: Distributed drugs for Vilmer, and was supposedly hired by Col. Styles to kill Sgt. West.- West: Found out about the drug operation 1 week prior to the events in the movie, and went to inform Col. Styles about the discovery. Because of Col. Styles' inaction and potential threat to his safety, Sgt. West arranged the training mission in order to preemptively eliminate Kendall and Mueller.- Col. Styles: Covered Vilmer's drug operation in order to make millions of dollars from the drug operation.Sgt. West found out that Vilmer was running a drug trafficking operation out of the hospital, and he also found out that Kendall and Mueller were involved in the operation. Not knowing that Col. Styles was involved in the operation, Sgt. West approached him to reveal the drug operation. After some time, the inaction of Col. Styles arose suspicion . Sgt West also suspected that Mueller and Kendall would be ordered to kill him.Sgt. West then arranges the training exercise in order to confront or eliminate Mueller and Kendall.