What is an operation in which a force moves forward or rearward through another force's combat positions with the intention of moving into or out of contact with the enemy?

The term "Passage of Lines" refers to a tactical maneuver where one unit moves through the positions of another unit that is already engaged with the enemy. This operation is essential for maintaining the flow of combat operations and ensuring that forces can reposition without disrupting the actions of friendly units. In this context, the advancing force either seeks to join or disengage from combat with the enemy, thus facilitating a coordinated movement on the battlefield.

This concept is crucial for operational success as it allows for greater flexibility in managing forces and can enable fresh units to take over an engagement, maintain the momentum of an attack, or reposition troops for better strategic advantages. The passage must be carefully coordinated to avoid friendly fire and confusion during the maneuver, emphasizing the importance of communication and situational awareness among units in contact.

In contrast, the other options represent different tactical concepts that do not fit the definition provided in the question. A frontal assault focuses on engaging enemy forces directly instead of moving through another unit's position; encirclement involves surrounding the enemy rather than passing through; and a flanking movement refers to attacking the enemy from the side rather than through another unit's combat positions.