It is evident enough that America can claim this title currently, but I would argue that it is applicable no matter how far back on the timeline you go. Certainly no other country or even empire on record has ever wielded the power we do now. We indisputably have the capability of conquering the entire world in almost a moment's notice, if only for the superhuman weapons array we possess. The only other claimant I have heard for greatest nation in the world is Rome. It seems like a fair enough claim, given that Rome did indeed conquer the known world at the time. To me, however, the fact that America, with the power it has, has not conquered the world shows more impressive greatness.
Rome was great and magnificent. But Rome was also thoroughly pagan, pagan in ways that America, with all the flaws and morally reprehensible customs has never been. Rome condoned infanticide, the father's right to demand an abortion of his pregnant wife, or even then sell the newborn child into slavery to earn money. Prostitution did not just exist, as it does here, but it was legal. It was even expected at certain religious festival in select temples. America's moral situation is hazy at best when it comes to sexual issues. Pornography and underground sex slavery does happen here. But doesn't it say something when it is still an enormous scandal when our elected officials are involved with such things? Involvement with prostitution for such an official can ruin a career for life.
Rome knew it was better than all its neighbors, and so felt morally justified and possibly even impelled to conquer them. America is sure of its golden opportunities and stands ready to welcome all those who wish to come in. "Bring me your weary, and your poor..." stands as another "Come to me all you who labor and are weary..." Rome never rode to anyone's rescue that I remember. But over the twentieth century, rescuing the weak could almost be mistaken for America's primary foreign policy. Korea, Vietnam, Kuwait, Somolia. Any tiny, helpless nation being attacked from within or without is not too tiny to merit America's attention. Any major catastrophe, be it tsunami, earthquake, or flood, who's there first? The United States Marine Corps. I find it hard to imagine the Roman Legions responding in like manner.
And that, ultimately, is why I believe America to be inexpressably greater than Rome. It is great in a Christian sense, not in a pagan sense. Perhaps no soldiers matched the legionaires of Rome in the millenia between them and America. But I know the Marines are a match for anyone, anywhere, anytime. And they are the ones with the order of the Tomb of the Unknown Soldier, the pin of which you cannot wear unless you refrain from drinking, swearing, and other immoral behavior for a year while guarding the tomb. What did Rome have to match that? They watched gladiators slaughter each other and wild animals tear apart Christians for entertainment. At least we are more addicted physical contact in sports such as football, where no one has died since they stopped wearing those leather helmets. Is this weakness? No. It is greatness.