What is a key purpose of machine guns in defense operations?

The key purpose of machine guns in defense operations is to retain or control terrain. Machine guns provide a significant advantage in defensive positions by delivering sustained and accurate firepower, which can deter or halt enemy advances. Their ability to cover specific sectors allows infantry units to establish effective fire lanes, creating a psychological and physical barrier against enemy forces.

In defense, the establishment of control over terrain is crucial because it allows friendly forces to take advantage of natural cover and obstacles. By using machine guns to create overlapping fields of fire, defenders can maximize their effectiveness in holding positions and protecting vital areas. This is critical during an engagement as it aids in maintaining the integrity of defensive lines and can significantly influence the outcome of combat operations.

The other options, while potentially relevant in different contexts or with different weapon systems, do not accurately capture the primary role of machine guns in a defensive strategy.