The U.S. at one time did seek to build and maintain an overseas empire, from about the mid-1890s, and then it relinquished most, but not all, of its imperial territories over the course of the 20th century, culminating in the return of the Panama Canal to Panama at the end of 1999. So imperialism is part of the U.S.'s past, and the trend is away from empire, not toward it.On imperialism, I agree with you that the US is not yet in an openly imperial mode. Rather, I compare it to the Roman Republic in the period leading up to Empire, when the economic and political forces were in extreme tension between traditional republican institutions and emerging military dictatorship. I hope the US never gets to the dictatorship stage, but the crazy scale of its military expenditure, together with the problem of the current financial collapse, suggests to me that this unlikely result is not as farfetched as you might think.
Respecting military expenditure, according to the versions of Statistical Abstract of the United States available at http://www.census.gov/compendia/statab/ , the ratio of U.S. military expenditure to GDP was 54% in 1944; 8% in 1970; 4.8% in 1980; 5.2% in 1990; 3% in 2000; and 4.5% in 2007, the latest year for which we appear to have accurate and complete U.S. GDP figures. These levels of military expenditure do not appear to me to be "crazy." The 2007 figure is lower than the figure at the height of the Vietnam War, lower than in 1980 during the Cold War and lower than in 1990 at the end of the Cold War. Over the 20th century the overall trend was downward; during the last 37 years the overall trend has been downward. I would not characterize this downward trend in military expenditure as "crazy."
Further, the U.S. military is under civilian control, exercised by the President of the United States, who is commander in chief of all U.S. military forces, and the U.S. Secretary of Defense. I see no evidence that such civilian control is threatened in any respect by any component of the U.S. armed forces.