The United States Military is considered by many historians to be the greatest combined-arms military in world history for many reasons.
First, it is one of the finest examples of a civilian controlled military in world history. A popularly elected executive is the head, but the army cannot be deployed for more than 60 days without the consent of an elected group of congressional representatives.
The U.S. military is also a mostly volunteer fighting force. Unlike other great armies in history which were made up of slaves or conscripts, the U.S. military has been a volunteer fighting force for most of its existence. Although conscription acts have been used in time of war, the armed forces try to maintain a strictly volunteer basis for its forces.
All of the branches of the military work together during combat operations under a single command, with the exception of the Coast Guard. This model, which allows each branch to support the other, gives the U.S. military a degree of flexibility that is seldom seen in other armies.
The military has also been a unifying force throughout U.S. History. Other armies have the reputation for being forces of suppression; the U.S. military has been a symbol of patriotic pride. The Founding Fathers were careful not to maintain a standing army early in our history to lessen the chances of military coups and occupation within our own borders.
Lastly, if you look at the size and scope of the modern U.S. military you couldn’t help but consider them a world-class military machine. 43% of all global military expenditures are spent on the U.S. military. Adding to this the caliber of their equipment and the bases they have around the globe, they could easily be considered as one of the finest combined-arms forces in world history.