How should you treat a casualty with an effective tourniquet who needs hypothermia treatment?

When treating a casualty with an effective tourniquet who requires hypothermia treatment, covering the casualty completely except for the face is the most appropriate action. This method helps retain body heat and prevents further heat loss, which is crucial in managing hypothermia.

By covering the casualty, one can create a controlled environment that minimizes exposure to the cold air, thus significantly improving their chances of recovering from hypothermia. It's important to keep the head and face exposed for monitoring and breathing purposes while ensuring other parts of the body are insulated to maintain warmth.

Other options, such as applying ice packs or exposing the casualty to warm air, would not effectively address the need to conserve body heat and could potentially worsen the condition. Likewise, while giving warm liquids may seem beneficial, it may not be feasible in all situations, especially if the casualty is unable to swallow or has reduced consciousness levels. Therefore, the chosen answer aligns best with the principles of efficiently managing hypothermia in a tactical environment.