What document initiates an amphibious operation by establishing mission and intent?

The document that initiates an amphibious operation by establishing the mission and intent is known as the initiating directive. This directive serves as the foundational communication that outlines the objectives and overarching purpose of the operation. It provides necessary guidance and direction to subordinate forces, ensuring all units are aligned in their understanding of the operation's goals and the actions they are to take. The initiating directive typically includes key details regarding the operational environment, timelines, and resources, allowing for effective planning and execution of the amphibious operation.

In contrast, the other documents mentioned have distinct purposes. A planning directive focuses on the detailed planning processes and tasks necessary to support the mission, while a command directive provides authoritative orders from command to execute specific actions. An operational directive, on the other hand, tends to outline broader operational guidelines but does not specifically initiate the mission in the way that an initiating directive does. Therefore, the initiating directive is integral for establishing mission clarity and intent, making it the correct answer.