Why aren't all branches of the military combined?

Hi there, I'm wondering about the organization of the military. Specifically, why aren't all the branches of the military combined into a singular entity? What purpose does having distinct branches serve?

The structure of the military into separate branches mainly serves two purposes: specialization and checks and balances.

Each branch of the military -- Army, Navy, Air Force, Marines, Coast Guard, and Space Force -- has its own specialized focus and responsibilities which require unique training, skills, and equipment. For instance, the Navy is primarily responsible for maritime operations, while the Air Force focuses on air and space superiority. The Army, on the other hand, is the primary land-based branch, and the Coast Guard carries out maritime law enforcement and search and rescue operations. Most recently, the Space Force was established to specifically deal with all things related to space, for instance, monitoring space debris and maintaining satellites.

These specializations allow each branch to develop and maintain expertise in their respective areas. By having specialized branches, the military can ensure that it is prepared for a wide variety of potential conflicts. In other words, it's an efficient way to organize a complex set of responsibilities.Moreover, with the military divided into separate branches, a system of checks and balances is implicitly created. No single entity has total control over all aspects of the military, contributing to the balance of power within the military and greater transparency. It also allows for internal competition and innovation, as each branch tries to stay ahead in its particular domain.

Of course, while they operate separately, the branches of the military often work together on joint operations and projects. The Department of Defense governs all branches, enforcing the overarching coordination among them.