Defense: The supremacy of US military can be challenged by China once China takes a policy decision to allow China money freely exchangeable like US$. US can get the support services of men and machine in places like Pakistan / Iraq by giving away US$ bills. China can give away US$ bills as well as China money. Giving away China money can buy more day to day needs at lower prices. Men work for money, tangible ones.
JOBS: Walmart can pay US$ bills to import goods. No china company can import goods with out earning exchange. So every US company should be exchange neutral , will ensure jobs are created in US for same value created abroad.
With computers allowing a global free trade as a free use of a nations currency internationally will detrimental to US supremacy.