What term describes a line or circle of police, soldiers, or guards preventing access to an area?

The term that describes a line or circle of police, soldiers, or guards preventing access to an area is "cordon." A cordon serves as a physical and strategic barrier that restricts entry to a specific zone, often used in law enforcement and military contexts to control the movement of people and ensure safety during various operations, such as crime scenes, public events, or emergency situations.

The effectiveness of a cordon lies in its ability to manage access while simultaneously allowing for controlled operations within or around the confined area. Typically, a cordon is implemented to safeguard the public or to ensure that a situation can be handled without outside interference. Other terms like "barrier," "access point," and "checkpoint" have distinct meanings and functions; for instance, a barrier generally refers to a physical obstruction, while a checkpoint often implies a location where thorough checks are performed, rather than an explicit line preventing access.