First, you are right that the USA is not yet an empire, although it has many of the characteristics of an empire. For example, we send our domestic police to hunt for criminals in other countries and station our troops in foreign lands. You will note that whereas we have troops in Japan and Germany, they have none in the US.
However, we will be an empire, born of chance and need. The need is that this is the only way to fight and win the war against Islamofascism. The war with Iraq will result in our conquest of that country. It’s people will be more free and will live better as a result, but we will be in charge of the government – even if someone acceptable to us becomes the de-facto ruler. We will occupy that country, and our troops will be able to occupy other areas in that part of the world that harbors our enemies.
I hate to disagree with you, but you may not understand the nature of previous empires. In many cases, they brought a better life to the populations of the countries they conquered. The British created their empire with military might, and enforced their rule with armed force, but they generally increased the amount of freedom the ordinary person enjoyed in the lands they conquered, abolished slavery, provided fairer government, introduced modern medicine, and in general improved the lot of those countries that were part of the Empire.