The Trump administration created the Space Force, to the delight of some and to the mockery of others. While I generally disagree with Trump on most matters, I do think that the creation of the Space Force was probably a good idea. This specific creation does raise general questions about when to unify and when to divide/create. I will, of course, look at the matter from a philosophical perspective.
From a practical standpoint, it makes sense to create a new organizational structure (such as the Space Force) when the proposed entity has a distinct purpose/mission that would warrant such a creation. Plato addressed a matter somewhat like this in the Ion when he discussed the spheres of various professions. The gist is that a profession is defined by the discipline one masters and Plato uses this to attack the poets’ claim that they possess knowledge they can teach. His argument is that poetry has no content of its own—for example, though a poet might write about medicine, that field belongs to the doctors. In the case of creating a new military branch by dividing it from an existing one, the question would be whether it has diverged enough from the distinct mission of the original to justify its existence as separate profession.
A good, albeit obvious, analogy is the creation of the United States Air Force on September 18, 1947. Prior to that date, the air forces were part of the United States Army. A quite reasonable justification for its creation was that the airplane had developed into a full-fledged weapon of war and there was a need for a large, but focused, military structure to command, control and conduct air operations. As such, a reasonable case could (and was) made for the need to create a distinct branch, the Air Force. While the United States is not operating combat spaceships, the complexity and scope of space warfare would seem to warrant the creation of the Space Force for reasons analogous to the creation of the Air Force. As such, if the creation of the Air Force was warranted, the creation of the Space Force also seems warranted. Fortunately, we did not have a space war to convince people that it is needed.
A second obvious analogy is to the Navy. It makes sense to have distinct military forces for different areas. The Army, in general, fights on the land while the Navy fights on the water. Of course, the Navy also operates on the ground (and in the air) and the Army operates in the air and on the water. But this broad distinction would seem to warrant the creation of the Space Force—so it can focus on space war. While this is no argument, my love of science fiction makes me inclined to approve of Space Force—I am accustomed to the fictional “existence” of things like Star Fleet and the High Guard.
There are, of course, principled reasons for not creating a new organization. One pragmatic one is that it would result in the creation of a new bureaucracy and inflict new administrative costs. In the case of the Space Force, it will presumably get its own space army of administrators, managers and bureaucrats on top of those who do space forcing. The question, as always, is whether the advantages of having a distinct organization outweigh the bureaucratic costs. Bureaucrats always think they do; which is why bloat infects every aspect of civilization. Some of this can be addressed by trying to keep the administration streamlined, which is always a challenge because of administrative creep (the tendency of administration to creep its way to an ever-expanding body of people, rules and paperwork). It does remain to be seen if the creation is worth the administrative cost and one must also consider redundancy. For example, the Army still operates its own aircraft despite the existence of the Air Force. So it is likely that the other forces will maintain and expand their space warfare capabilities as well, perhaps doubling the cost of having a specific capability. In some cases this can be justified and is something that would need to be addressed on a case by case basis. But, in general, I would say that the existence of the Space Force is warranted.