One thing that George Friedman points out about the US is that while we are excellent at fighting wars, both of limited and vast scope, we struggle immensely with counter insurgency. This ties into what Vox wrote about, though it should be mentioned we have won many wars in the latter have of the 20th and 21st century. We defeated Saddam twice, out maneuvered the Soviet Union, unseated communists in Grenada, and bent the will of Serbia. Many of these actions are not considered wars, but the fact is that the United State utilized force to achieve an end. And that is all that war is, whether you fight for freedom, gold, land, oil, or power, you (the nation) are simply using violence to obtain that end.
The US does do a very poor job at counter insurgency, but most nations track records are hardly any better. It is difficult to win, primarily, as George Friedman states, you are often fighting a foe who is far more motivated than you and is also a part of the native population. While many governments, such as Mexico, cannot defeat their insurgents, the drug lords, due to political corruption, and others, such as Mali, cannot win due to a lack of equipment, there are still other nations, such as Afghanistan, where the fighting force holds a mechanical advantage, due to US aid, but the rank and file does not have the same conviction.
The US doesn't suffer from most of these problems. Our military is highly trained, motivated, and well equipped. Corruption isn't a huge issue in our military, in fact it is the last government institution that many Americans still respect, but the US does have an issue. And that is a national one, force of will. The United States, for all its faults, actively tries to be a benevolent force and influence for the most part. One, that while it wants hegemony, so far seems to eschew empire.
Contrast this with Rome. Rome initially was very similar to the US in terms of foreign policy. During its republican, and for some during its imperial, years many lands that are now shown in maps as part of the empire were in fact lands and kingdoms that were largely internally independent. These lands enjoyed a degree of independence and self determination that varied. Some lands governed themselves entirely but often sided with Rome for a variety of reasons, think of Japan or Germany, others largely governed themselves but saw their international policy dictated to them, think of Puerto Rico, and still others were integral parts of the empire, like ours states today. However as time went on this changed.
As the imperial years went on more territory was formally incorporated into the empire. This did cause some rebel and insurgent movements. But considering that the empire lasted for five years in the west, and almost a thousands years if you include the east, you can conclude they were largely effective at putting insurgencies down and conquering other nations? What made Rome so effective where the US struggles? Brutality, use and simple.
The Romans had no compunction in conquering nations. By conquering I mean culling and displacing entire populations selling of the survivors that resisted into slavery. Couple this with the tremendous economic advantages that could be acquired by siding with Rome and it made it very difficult to side with rebel forces. There was no hand wringing when it came to this. Family members of suspected fighters could possibly find themselves enslaved, and villages that resisted completely annihilated. Carthage, a rival to Rome at one point, was completely leveled to the ground. The men slaughtered with the women and children being flung across the corners of the empire as slaves. For good measure the Romans sowed the land with salt, a symbolic gesture meaning that nothing fertile would ever grow here again. The US has done things like this in the past, particularly when it came to settling the west. But it was and is horribly bloody and these actions often run against the grain of values, especially when it comes to the value of human life, that democratic nations posses.
This, the system of values of the US possesses, shows in the unwillingness of the US to actively conquer other nations. The US does invade and occupy, but it doesn't conquer. There is a difference between the two. For the most part the US doesn't need to do so to achieve its ends. The US uses the time during its occupation to build political and economic networks that it can use later. This isn't unusual, as you can conquer a nation and do this as well, but once you leave a nation that merely has been occupied the nation will largely do whats in its own best interest. Look at Iraq, we have occupied them for nearly a decade, built a new military dependent on our surplus equipment, and established a new political system. By all accounts you would think that Iraq was a satellite of the US save for the fact that as we have pulled out the nation moves closer and closer to Iran.
A conquered nation would never do this, or it would take a large amount of external pressure for it to do so. Case and point look at Japan. We conquered Japan, perhaps the nation in the 20th century that the US truly conquered. But look at what we did. We slowly strangled Japan economically. We cut off their oil supplies from the East Indies and methodically destroyed their infrastructure. We firebombed Tokyo, killing more civilians than either Hiroshima or Nagasaki. After dropping atomic hellfire on the people of Japan we demanded unconditional capitulation, and threatened to wipe their nation off of the map if they didn't. The Japanese were convinced that we were prepared to utterly destroy their civilization, and if we had possessed any more nuclear weaponry, we might have continued to bombard them. We were prepared to destroy them, and more importantly, we were also willing to bleed.
It isn't talked about outside of historical circles, but the US was preparing for an invasion of Japan. If the US had not invented or used atomic weaponry it would have been the one of the largest military operations to date. It would have been very bloody. Millions of Japanese would have died. And the median estimate would have put the death toll for Americans around one hundred to three hundred thousand. That is a steep price to pay. One that we would not pay now, nor would most nations as well.And that is the final crux of it. To conquer a nation, to win a war where the end goal is to completely mold and shape a nation how you desire to shape it, requires the willingness to spill blood to the extreme.
The US doesn't desire to conquer as our geographic goals have already been meet and politically it would be distasteful. I don't say that our unwillingness to do so is a bad thing. True empire holds little allure to me, especially considering the terrible human toll that is required. Our recent wars could have been very limited. The US could simply have concerned itself with Al-Qaeda in Afghanistan, and we could have simply killed Saddam and threatened the new leaders if preventing development of WMDs were simply our goal. But the US took these events, some out of circumstance and some out of choice, to try and mold these nations like the US molded Japan, Germany, and many others. The US was doomed to failure because without the willingness to conquer and since there were no other external influences, such as the Soviet Union, that would have deterred domestic rebellion or fostered an equally committed counterinsurgency amongst the local populace, there is no chance of us outlasting an insurgency in their homeland.