US Air Force
Can't be done. The price would rise to the point where the value would exceed the entire US economy long before they could obtain 51% of the currency (control). That's if they could convince the dispersed holders of the 75% in circulation to sell. By nature, it is decentralized and fluid. Also, disagreements can fork the currency (thanks for the free money, forkers)Until the central banks either buy it all, or sabotage it. Likely they'll do the latter, then the former.