Whats wrong with a American Empire?

We are the only great power in history that has not leveraged its power to expand. If we use our military as a conquering force rather then police we would have won vietnam and would have already won afghan/Iraq. Even if afghan/Iraq gets their act together in democracy, there is no guarantee they wont elect a Taliban extremist leader. The countries could do much worse then to be annexed into the USA. We would get lower oil prices and create a model of a good stable community to the surrounding nations. The surrounding nations would behave themselves for fear of their own annexation. Isn't like haven't used this method before. How do you think we got most of our southern border-states and California? If we look at the native americans we annexed all our nation from there's. Its not a radical american idea.