Fear: After the collapse of the Soviet Union, there is no longer another nation with the military might to put the United States in check. A power vacuum was created, and theoretically the United States could become a juggernaut and go trouncing around the world, taking over other countries at will.
I have read a few comments here where all "super powers" in the past have eventually fallen. Rome, the Ottoman Empire, Khan in Asia, Ancient Egypt, Napoleonic France just to name a few...
However, if you examine these past fallen "Empires", each one involved a conquering nation trying to spread it's physical borders to gain influence. Occupying defeated nation after defeated nation eventually stretched the occupier's military beyond it's capacity to quell any uprisings or resistance.
The United States is not an occupying nation. We have no desire to "divide and conquer", nor do we wish to spread our physical borders throughout the world. Our citizens wouldn't tolerate it, as it goes against our core beliefs in freedom.
We are willing to sacrifice our own citizen's blood to defend other nations who are powerless to defend themselves. The vast majority of Americans are a good and honest people who genuinely care about the welfare of others. We will also fight to protect our own interests. We have earned that right.
Many countries with Anti-United States sentiments enjoy the freedoms they have today because of us. So go ahead and keep bashing away at the United States... Unlike the Romans, we will still be here to tolerate your short lived gratitude now turned to disdain. We won't hang you for expressing your hatred, nor will we come rolling in and occupy your country and make you wave American flags.