The highest ranking officer in the United States armed forces, the Chairman of the Joint Chiefs of Staff,  succeeds at his job via begging and deal making skills, not because of any particular military talents. The chairman is the senior military officer, but has little in the way of real power and resources. 

The Chairman of the Joint Chiefs of Staff is the presidents chief military advisor, and the chairman for a committee comprising the Chiefs of Staff for the four military services.  These Chiefs of Staff are very much in charge of their own services, and have a large annual budget (over $50 billion each, even in a bad year). But the Chairman has no money, which causes lots of problems because of the current drive to develop more joint (two or more services working together) activities. The Chairman has to convince the service chiefs (of staff) to come up with money for joint projects. The Secretary of Defense has control over billions of dollars of Department of Defense money, but the purpose of jointness is to get the services to commit their own money and people to projects that benefit everyone. Some proposed jointness projects are obvious, like having one basic flight school for helicopter pilots, and another for fixed wing pilots. The basics are the basics, and the services can still have specialized flight schools for advanced training on the aircraft unique to each service. But the services resist this sort of thing, partly out of tradition, party out of a reluctance to giving up money and authority. 

The United States purposely avoided having a supreme military commander who was a member of the military. The president of the United States is, by law, the commander in chief of the armed forces. In theory, the president, as commander in chief, could simply order the service chiefs to participate in these joint projects. But the presidents authority is not absolute, and is encumbered by laws, customs, and the willingness of Congress and the Supreme Court to side with service Chiefs who want to resist an order from the commander in chief. So it usually falls to the Chairman to try and convince the service Chiefs, and their Congressional allies (who are very interested in where all those defense dollars are spent) to go along with any changes in the status quo. 

The creators of the American constitution and government deliberately wanted to avoid the risk of a military coup. For thousands of years, such military takeovers had been a real threat worldwide. So in America the command of the military is purposely split into many different pieces. While cooperation among the various parts of the military is seen as a good thing, the laws and traditions that govern American military organization deliberately make it difficult for the armed forces to act in a unified fashion. This is not likely to change any time soon.