Military Tradition and Innovation
At the height of its power, ancient Rome kept its power and influence by having one of the finest military machines the world has ever seen. Its battlefield tactics led to victory after victory, designed specifically to disrupt and destroy every army it faced. On top of this, Roman engineering skills were beyond compare, allowing them to effectively siege and overcome any city or fortification put in their way, nullifying any defensive advantage the enemy would seek to gain for themselves. Perhaps most importantly, Rome was extremely good at making allies in the region that they would fight in, promising monetary or territorial reward in exchange for a reliable base of supply. Lastly, Roman legionaries were rewarded well for their hard service; it wasn't uncommon for legionaries to be given sizable grants of land and booty after a campaign abroad. Thus was the incentive created for Romans to be all they could be; with great discipline came great reward, a reward that was never ending so long as Rome continued to expand.
Which of course it stopped doing at a certain point. Once the promise of exotic bounty abroad was removed, it became difficult for Rome to create and sustain the disciplined and experienced armies that it was used to. Obtaining loot beyond the normal legionary salary became rarer and, generally, only found when the legions were engaged in civil war (which was disturbingly often and further contributed to erosion of Roman military tradition). Privatization of the army occurred to pick up the manpower slack; mercenaries were brought in from Germania and Asia Minor to fill the gaps. But with this privatization came a reduction in discipline and loyalty to home. Military innovation was shrugged off as irrelevant, which meant that, in time, Rome's strategies only allowed an even keel with their foes. Their distinct military advantage was lost as their enemies adopted the Roman military method. This meant that legions, while still formidable, became beatable, which removed their mystique and made losses far more common. This, along with the difficulty of defending such a sprawling empire, contributed prominently in Rome's eventual downfall.
The United States since World War II has been considered the most powerful and technologically advanced military force the world has ever seen. While many people question whether this will remain so, even today it is impossible to point to any other country in the world who could defeat the U.S. military in a straight engagement. The United States proved this by trouncing the Iraq army in Operation Desert Storm so thoroughly that it has seen no challenge since. Much like ancient Rome's enemies, the United States' foes have learned that one can't stand up to the superpower and win, one must fight it indirectly through terrorist acts and guerrilla warfare.
But insurgencies and terrorism did not bring down the Roman empire. It is true that a good deal of military and civilian resources had to be devoted to maintaining stability but, like America today, Rome managed to overcome a great deal of that through the sharing of a dynamic culture. In ancient Rome, this manifested by giving their conquered provinces a voice and making it fairly easy to become a Roman citizen with all the benefits that came with it. In the United States, we don't have “conquered provinces”, but it is clear that our focus on human rights and democracy is immensely attractive to the world at large. When Rome turned inward, it lost that appeal and thus lost its position as the central power of its time. So long as the United States maintains its emphasis on democratic rights and freedoms, I doubt it will go the way of Rome in that regard.
Military expenditure by top five nations
Privatization of the United States military is a concern though. It is worth noting that use of private military corporations and contractors are at their highest level in the United States, and rising. There has already been concern as to their accountability and adherence to United States law. If the lessons of ancient Rome are of any guide, this development is a concerning one.
The constant desire to reduce and/or outsource military research could be disquieting also. Especially in today's modern warfare, the quantity of troops matter nowhere near as much the quality, technology, and kit assigned to them. The use of drones and cyber-warfare are both immense innovations with which the United States is doing its best to remain the head of the pack. This, perhaps, is an argument to leave the defense budget alone. But a better argument would be to make it leaner and more efficient. Much like elephants and chariots became obsolete in ancient Roman warfare, modern battle tanks seem to have no place in an arena where they can easily be bombed from the sky or rendered irrelevant by complicated urban warfare. So long as the United States invests wisely in the tools it uses to help maintain peace and stability throughout the world, it is unlikely to suffer the erosion of faith and capability that the Romans underwent so long ago.