Under whose authority can forces be transferred once assigned to a unified combatant command?

Forces assigned to a unified combatant command can only be transferred under the authority of the Secretary of Defense or through special procedures requiring the approval of the President. This is rooted in the principles of command authority and the operational control exercised over military forces within the unified command structure.

The Secretary of Defense holds the ultimate responsibility for the management and employment of military forces, ensuring that any transfer aligns with national security policies and operational requirements. Furthermore, special procedures that involve the President's approval allow for exceptional cases that may require the highest level of authority to enact changes, reflecting the importance of strategic decision-making in military operations.

This authority is vital for maintaining organized military response capabilities and ensuring that forces are effectively utilized in alignment with national defense objectives. The other options, while holding significant roles within the military and defense policy, do not possess the same level of authority regarding the transfer of forces assigned to a unified combatant command, as they operate within different scopes of command and responsibility.