It is interesting to remember the key point of Paul Kennedy's book "The Rise and Fall of the Great Powers" - namely that the sheer cost of maintaining the military power required to hold on to power is what eventually weakens and destroys powerful nations.
This sound like a theory that could have been put forward in the year 1400 as well, when the role of the king was just the upkeep of his own household and his army, but the point of Kennedy's book is that it remains true even for more modern regimes. When the Soviet regime collapsed it became clear that an unsound proportion of the GDP was pumped into the military, so the next question remains: Will this apply to the United States also?

The recently approved cost of approximately $80bn for the "War against terror" - mainly the fight against Iraq, adds 30%-50% to the budget deficit. The economy is weak already, and it is no entirely clear that America wil be able to foot this bill without dire economic consequences.
Add to that the recent turn in american politics reducing the Secretary of State to a more marginal role and focusing on America's ability to act with force. If the Iraq war cannot be contained in Iraq, of if success in Iraq proves to Bush that massive military action is a quick and effective way to deal with foreign policy then we could be at a world-order toppling economic disaster.