3-17. Security operations—screen, guard, cover, area security, and local security—are essential during preparation. During preparation, the force is vulnerable to surprise and enemy attacks. Leaders are often away from their units and concentrated together during rehearsals. Parts of the force could be moving to task- organize. Required supplies may be unavailable or being repositioned. Units assigned security missions execute these missions while the rest of the force prepares for the overall operation. Every unit provides local security to its own forces and resources.