What form of maneuver is used when a small force infiltrates enemy defenses to seize terrain undetected?

The form of maneuver referred to in the question is infiltration. Infiltration involves small units or forces moving through enemy lines and defenses to accomplish specific objectives, such as seizing terrain or engaging in actions behind enemy fortifications, all while minimizing detection.

The goal of infiltration is to maintain stealth and surprise, allowing the infiltrating unit to exploit weaknesses in the enemy's defense without engaging in direct confrontation. This tactic is particularly effective for units seeking to disrupt enemy operations, gather intelligence, or create a tactical advantage by positioning themselves in a way that is unexpected by the enemy.

In contrast, a flanking maneuver is aimed at attacking the enemy's side or rear, requiring more direct engagement with strong enemy forces. An envelopment involves surrounding an enemy force to cut off their retreat and supply lines, which typically requires larger forces. A direct assault is a straightforward attack on enemy positions, which often draws attention and might not achieve the element of surprise that infiltration offers.

Thus, infiltration is the most appropriate tactic for the scenario described, as it emphasizes stealth and the ability to take action undetected behind enemy lines.