U. s. law, title 10, usc section 153, gives the authority regarding joint force development, specifically providing authority to develop doctrine for the joint employment of the armed forces, and to formulate policies for the joint training of the armed forces to include policies for the military education and training of members of the armed forces

I think the answer is: Chairman of the Joint Chiefs of Staff
This section is really important because In real-life scenario, many situations may arise that require the combination more than one military branches. For example, in military battles that happened in the middle of Ocean, the combination of both Air force and the Marine corp is crucial to achieve the upper hand in the battle.