What is the first action to take when assisting casualties during care under fire?

When assisting casualties during care under fire, the first action to take is to apply a tourniquet to control bleeding. In a combat environment where there's an ongoing threat, the priority is to stop any life-threatening bleeding that could lead to rapid deterioration of a casualty's condition. Applying a tourniquet effectively minimizes blood loss and stabilizes the casualty for further treatment once the immediate threat is managed.

This action is critical because if bleeding is not controlled quickly, the casualty may not survive, regardless of other interventions. The tourniquet should be applied to the extremity above the injury site to maximize its efficacy. This imperative response prioritizes the severe risk presented by uncontrolled hemorrhage during high-stress and dangerous situations.

While assessing the environment for safety or checking the casualty's responsiveness are also crucial components of casualty care, in the context of active threats, managing severe bleeding takes precedence. Therefore, understanding the urgency of controlling hemorrhage first is integral to effective tactical combat casualty care.