In what phase of an amphibious operation do units engage with the enemy?

The phase of an amphibious operation in which units engage with the enemy is the Assault phase. During this critical stage, forces execute the plans laid out during the previous planning and rehearsal phases and physically land on the enemy's territory, initiating combat operations. This engagement typically occurs after units have conducted shoreline reconnaissance and have secured a foothold, allowing for the transition from sea to land.

In contrast, the Planning phase is focused on developing strategies and logistics required for the operation, including intelligence gathering and resource allocation. The Embarkation phase involves the actual deployment of troops and equipment from ship to shore, but without direct engagement. The Rehearsal phase serves to practice the operation to ensure readiness for the assault but does not involve live enemy engagement. Thus, the Assault phase is specifically when combat with the enemy begins, making it the correct answer.